[tsan] Fix the memcpy interceptor to be memmove compatible on OS X

On OS X, memcpy and memmove are actually aliases of the same implementation, which means the interceptor of memcpy is also invoked when memmove is called. The current implementation of the interceptor uses `internal_memcpy` to perform the actual memory operation, which can produce an incorrect result when memmove semantics are expected. Let's call `internal_memmove` instead.

Differential Revision: http://reviews.llvm.org/D14336

llvm-svn: 252162