Don't manually define WIN32/WIN64

All the code that relies on separating them uses the built-in defines
_WIN32 and _WIN64, or the corresponding machine defines (such as
_M_IX86, for MSVC 32 bit inline assembly).


Also revert "Set ASM_ARCH in all cases to fix Win64 Debug build"
    
There is no need to set ASM_ARCH unless assembly sources actually
are built. ASM_ARCH is no longer used for setting unrelated CFLAGS.