runtime: check and fail early with a message if MMX is not available on 386
runtime: check and fail early with a message if MMX is not available on 386

Fixes #12970.

Change-Id: Id0026e8274e071d65d47df63d65a93110abbec5d
Reviewed-on: https://go-review.googlesource.com/15998
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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