[INTRIN]

[INTRIN]
- Implement _mm_sfence, _mm_lfence, __faststorefence (for amd64)
- Don't use __sync_synchronize() for _ReadWriteBarrier, as it issues an mfence instruction and this is not what we want
- Remove "BUGBUG" comment, because the fact that _ReadBarrier and _WriteBarrier are full (compiler) barriers isn't critical.

svn path=/trunk/; revision=50456