Make MemoryUtil.cpp use the correct x86_64 define.

MemoryUtil.cpp was incorrectly using the old __x86_64__ define when it should be using _M_X86_64.
It was also using _ARCH_64 when it shouldn't have which was causing an errant PanicAlert to come up in my development.
Merge pull request #446 from Sonicadvance1/Memoryutil-fix

Make MemoryUtil.cpp use the correct x86_64 define.