[X86] Fix prolog/epilog mismatch for stack protectors on win32-macho.

The xor'ing behaviour is only used for msvc/crt environments, when we're targeting
macho the guard load code doesn't know about the xor in the epilog. Disable xor'ing
when targeting win32-macho to be consistent.

Differential Revision: https://reviews.llvm.org/D71095