[MSAN] Instrument libatomic load/store calls

These calls are neither intercepted by compiler-rt nor is libatomic.a
naturally instrumented.

This patch uses the existing libcall mechanism to detect a call
to atomic_load or atomic_store, and instruments them much like
the preexisting instrumentation for atomics.

Calls to _load are modified to have at least Acquire ordering, and
calls to _store at least Release ordering. Because this needs to be
converted at runtime, msan injects a LUT (implemented as a vector
with extractelement).

Differential Revision: https://reviews.llvm.org/D83337