runtime: use uintptr for block length in scanblock
runtime: use uintptr for block length in scanblock

Using an int64 for a block size doesn't make
sense on 32bit platforms but extracts a performance
penalty dealing with double word quantities on Arm.

linux/arm

benchmark                 old ns/op    new ns/op    delta
BenchmarkGobDecode        155401600    144589300   -6.96%
BenchmarkGobEncode         72772220     62460940  -14.17%
BenchmarkGzip               5822632      2604797  -55.26%
BenchmarkGunzip              326321       151721  -53.51%

benchmark                  old MB/s     new MB/s  speedup
BenchmarkGobDecode             4.94         5.31    1.07x
BenchmarkGobEncode            10.55        12.29    1.16x

R=golang-dev, rsc, bradfitz
CC=golang-dev
http://codereview.appspot.com/6272047

mhennings/marcohennings-go,sanjosh/sanjos100-tipc,webfd/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,webfd/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,scirelli/scirelli-go,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,scirelli/scirelli-go,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,webfd/go-zh,Triskite/willstone-goclone,bryanxu/go-zh,Triskite/willstone-goclone,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,mhennings/marcohennings-go,Triskite/willstone-goclone,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,scirelli/scirelli-go,mhennings/marcohennings-go,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof