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
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go