sync/atomic: replace MFENCE with LOCK XADD MFENCE was introduced only on the Pentium4 (SSE2), while XADD was introduced on the 486. Fixes issue 2268.
sync/atomic: replace MFENCE with LOCK XADD
MFENCE was introduced only on the Pentium4 (SSE2),
while XADD was introduced on the 486.
Fixes issue 2268.

R=golang-dev, rsc
CC=fshahriar, golang-dev
http://codereview.appspot.com/5056045

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