[Intrin.h] Use compiler builtins to model memory barriers

_ReadBarrier, _WriteBarrier, and _ReadWriteBarrier are essentially
memory barriers of one form or another.  Model these as
atomic_signal_fence(ATOMIC_SEQ_CST).

__faststorefence is a curious intrinsic.  It's single purpose seems to
an alternative to mfence when that instruction is slow.  However, mfence
is not always slow and is, in general, preferable to a 'lock or'
sequence on certain CPUs.  Give the compiler freedom to select the best
sequence to get a fence.

llvm-svn: 242378