runtime: disable prefetching on 386
runtime: disable prefetching on 386

It doesn't seem to help on modern processors and it makes Go impossible to run
on Pentium MMX (which is the documented minimum hardware requirement.)

Old is with prefetch, new is w/o. Both are compiled with GO386=sse2.
Benchmarking is done on Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz.

name                     old time/op    new time/op    delta
BinaryTree17-4              2.89s ± 2%     2.87s ± 0%    ~     (p=0.061 n=11+10)
Fannkuch11-4                3.65s ± 0%     3.65s ± 0%    ~     (p=0.365 n=11+11)
FmtFprintfEmpty-4          52.1ns ± 0%    52.1ns ± 0%    ~      (p=0.065 n=10+9)
FmtFprintfString-4          168ns ± 0%     167ns ± 0%  -0.48%   (p=0.000 n=8+10)
FmtFprintfInt-4             167ns ± 0%     167ns ± 1%    ~      (p=0.591 n=9+10)
FmtFprintfIntInt-4          295ns ± 0%     292ns ± 0%  -0.99%   (p=0.000 n=9+10)
FmtFprintfPrefixedInt-4     327ns ± 0%     326ns ± 0%  -0.24%  (p=0.007 n=10+10)
FmtFprintfFloat-4           431ns ± 0%     431ns ± 0%  -0.07%  (p=0.000 n=10+11)
FmtManyArgs-4              1.13µs ± 0%    1.13µs ± 0%  -0.37%  (p=0.009 n=11+11)
GobDecode-4                9.36ms ± 1%    9.33ms ± 0%  -0.31%  (p=0.006 n=11+10)
GobEncode-4                7.38ms ± 1%    7.38ms ± 1%    ~     (p=0.797 n=11+11)
Gzip-4                      394ms ± 0%     395ms ± 1%    ~     (p=0.519 n=11+11)
Gunzip-4                   65.4ms ± 0%    65.4ms ± 0%    ~     (p=0.739 n=10+10)
HTTPClientServer-4         52.4µs ± 1%    52.5µs ± 1%    ~     (p=0.748 n=11+11)
JSONEncode-4               19.0ms ± 0%    19.0ms ± 0%    ~      (p=0.780 n=9+10)
JSONDecode-4               59.6ms ± 0%    59.6ms ± 0%    ~      (p=0.720 n=9+10)
Mandelbrot200-4            4.09ms ± 0%    4.09ms ± 0%    ~      (p=0.295 n=11+9)
GoParse-4                  3.45ms ± 1%    3.43ms ± 1%  -0.35%  (p=0.040 n=11+11)
RegexpMatchEasy0_32-4       101ns ± 1%     101ns ± 1%    ~     (p=1.000 n=11+11)
RegexpMatchEasy0_1K-4       796ns ± 0%     796ns ± 0%    ~      (p=0.954 n=10+8)
RegexpMatchEasy1_32-4       110ns ± 0%     110ns ± 1%    ~      (p=0.289 n=9+11)
RegexpMatchEasy1_1K-4       991ns ± 0%     991ns ± 0%    ~      (p=0.784 n=10+8)
RegexpMatchMedium_32-4      131ns ± 0%     130ns ± 0%  -0.42%   (p=0.004 n=11+9)
RegexpMatchMedium_1K-4     41.9µs ± 1%    41.6µs ± 0%    ~      (p=0.067 n=11+9)
RegexpMatchHard_32-4       2.34µs ± 0%    2.34µs ± 0%    ~     (p=0.208 n=11+11)
RegexpMatchHard_1K-4       70.9µs ± 0%    71.0µs ± 0%    ~      (p=0.968 n=9+10)
Revcomp-4                   819ms ± 0%     818ms ± 0%    ~     (p=0.251 n=10+11)
Template-4                 73.9ms ± 0%    73.8ms ± 0%  -0.25%  (p=0.013 n=10+11)
TimeParse-4                 414ns ± 0%     414ns ± 0%    ~     (p=0.809 n=11+10)
TimeFormat-4                485ns ± 0%     485ns ± 0%    ~      (p=0.404 n=11+7)

name                     old speed      new speed      delta
GobDecode-4              82.0MB/s ± 1%  82.3MB/s ± 0%  +0.31%  (p=0.007 n=11+10)
GobEncode-4               104MB/s ± 1%   104MB/s ± 1%    ~     (p=0.797 n=11+11)
Gzip-4                   49.2MB/s ± 0%  49.1MB/s ± 1%    ~     (p=0.507 n=11+11)
Gunzip-4                  297MB/s ± 0%   297MB/s ± 0%    ~     (p=0.670 n=10+10)
JSONEncode-4              102MB/s ± 0%   102MB/s ± 0%    ~      (p=0.794 n=9+10)
JSONDecode-4             32.6MB/s ± 0%  32.6MB/s ± 0%    ~       (p=0.334 n=9+9)
GoParse-4                16.8MB/s ± 1%  16.9MB/s ± 1%    ~     (p=0.052 n=11+11)
RegexpMatchEasy0_32-4     314MB/s ± 0%   314MB/s ± 1%    ~     (p=0.618 n=11+11)
RegexpMatchEasy0_1K-4    1.29GB/s ± 0%  1.29GB/s ± 0%    ~     (p=0.315 n=10+10)
RegexpMatchEasy1_32-4     290MB/s ± 1%   290MB/s ± 1%    ~     (p=0.667 n=10+11)
RegexpMatchEasy1_1K-4    1.03GB/s ± 0%  1.03GB/s ± 0%    ~      (p=0.829 n=10+8)
RegexpMatchMedium_32-4   7.63MB/s ± 0%  7.65MB/s ± 0%    ~     (p=0.142 n=11+11)
RegexpMatchMedium_1K-4   24.4MB/s ± 1%  24.6MB/s ± 0%    ~      (p=0.063 n=11+9)
RegexpMatchHard_32-4     13.7MB/s ± 0%  13.7MB/s ± 0%    ~     (p=0.302 n=11+11)
RegexpMatchHard_1K-4     14.4MB/s ± 0%  14.4MB/s ± 0%    ~      (p=0.784 n=9+10)
Revcomp-4                 310MB/s ± 0%   311MB/s ± 0%    ~     (p=0.243 n=10+11)
Template-4               26.2MB/s ± 0%  26.3MB/s ± 0%  +0.24%  (p=0.009 n=10+11)

Update #12970.

Change-Id: Id185080687a60c229a5cb2e5220e7ca1b53910e2
Reviewed-on: https://go-review.googlesource.com/15999
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

suncycheng/go,sndnvaps/go,roth1002/go,nawawi/go,binarycrusader/go,deft-code/go,goodlang/good,josharian/go,Jordanzuo/go,Samurais/go,Endika/go,theif519/go,momchil-velikov/go,Cofyc/go,linux-on-ibm-z/go,fjballest/golang,nawawi/go,roth1002/go,4ad/go,akutz/go,momchil-velikov/go,CAFxX/go,vsekhar/elastic-go,binarycrusader/go,goodlang/good,deft-code/go,kim-racktop/go,suncycheng/go,mwhudson/go,nawawi/go,momchil-velikov/go,ImJasonH/go,Jordanzuo/go,josharian/go,fjballest/golang,miolini/go,cloudflare/go,codestation/go,sndnvaps/go,cyberroadie/go,binarycrusader/go,suncycheng/go,derekmarcotte/go,derekmarcotte/go,odeke-em/go,cyberroadie/go,momchil-velikov/go,golang/go,Samurais/go,nawawi/go,akutz/go,Endika/go,binarycrusader/go,sndnvaps/go,4ad/go,Cofyc/go,miolini/go,vanloswang/go,miolini/go,golang/go,Samurais/go,Samurais/go,CAFxX/go,linux-on-ibm-z/go,akutz/go,derekmarcotte/go,4ad/go,cloudflare/go,linux-on-ibm-z/go,fjballest/golang,Endika/go,kim-racktop/go,kim-racktop/go,cloudflare/go,kim-racktop/go,polyverse-security/go,kim-racktop/go,fjballest/golang,vanloswang/go,odeke-em/go,nawawi/go,deft-code/go,miolini/go,suncycheng/go,Cofyc/go,theif519/go,binarycrusader/go,polyverse-security/go,CAFxX/go,momchil-velikov/go,goodlang/good,polyverse-security/go,ImJasonH/go,kim-racktop/go,Endika/go,derekmarcotte/go,suncycheng/go,sndnvaps/go,codestation/go,ganboing/go-esx,vanloswang/go,4ad/go,ganboing/go-esx,cloudflare/go,roth1002/go,suncycheng/go,derekmarcotte/go,vanloswang/go,vanloswang/go,odeke-em/go,josharian/go,fjballest/golang,sndnvaps/go,ganboing/go-esx,christopher-henderson/Go,cyberroadie/go,Jordanzuo/go,nawawi/go,vsekhar/elastic-go,ImJasonH/go,odeke-em/go,cyberroadie/go,Cofyc/go,deft-code/go,Endika/go,fjballest/golang,momchil-velikov/go,polyverse-security/go,CAFxX/go,theif519/go,akutz/go,binarycrusader/go,vsekhar/elastic-go,miolini/go,ganboing/go-esx,cyberroadie/go,goodlang/good,Endika/go,derekmarcotte/go,vsekhar/elastic-go,linux-on-ibm-z/go,CAFxX/go,golang/go,sndnvaps/go,derekmarcotte/go,ganboing/go-esx,linux-on-ibm-z/go,vanloswang/go,codestation/go,vsekhar/elastic-go,deft-code/go,mwhudson/go,josharian/go,miolini/go,odeke-em/go,Samurais/go,golang/go,linux-on-ibm-z/go,goodlang/good,mwhudson/go,cyberroadie/go,josharian/go,golang/go,polyverse-security/go,Endika/go,CAFxX/go,ImJasonH/go,roth1002/go,kim-racktop/go,josharian/go,cyberroadie/go,vsekhar/elastic-go,linux-on-ibm-z/go,goodlang/good,Jordanzuo/go,deft-code/go,momchil-velikov/go,cloudflare/go,ImJasonH/go,theif519/go,momchil-velikov/go,odeke-em/go,akutz/go,CAFxX/go,fjballest/golang,miolini/go,binarycrusader/go,sndnvaps/go,Jordanzuo/go,ganboing/go-esx,theif519/go,golang/go,4ad/go,polyverse-security/go,theif519/go,vsekhar/elastic-go,ImJasonH/go,theif519/go,binarycrusader/go,akutz/go,odeke-em/go,nawawi/go,roth1002/go,mwhudson/go,odeke-em/go,codestation/go,Cofyc/go,akutz/go,cloudflare/go,Samurais/go,4ad/go,vanloswang/go,mwhudson/go,sndnvaps/go,fjballest/golang,akutz/go,vanloswang/go,golang/go,ganboing/go-esx,polyverse-security/go,theif519/go,cloudflare/go,derekmarcotte/go,Cofyc/go,cyberroadie/go,mwhudson/go,goodlang/good,josharian/go,golang/go,4ad/go,josharian/go,suncycheng/go,deft-code/go,cloudflare/go,roth1002/go,Jordanzuo/go,Samurais/go,codestation/go,polyverse-security/go,codestation/go,Cofyc/go,Endika/go,ImJasonH/go,roth1002/go,goodlang/good,ImJasonH/go,Cofyc/go,Samurais/go,Jordanzuo/go,mwhudson/go,deft-code/go,Jordanzuo/go,mwhudson/go,codestation/go,linux-on-ibm-z/go,4ad/go,codestation/go,miolini/go,kim-racktop/go,CAFxX/go,ganboing/go-esx,suncycheng/go,vsekhar/elastic-go,roth1002/go,nawawi/go