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).
Revert "Set ASM_ARCH in all cases to fix Win64 Debug build"

This reverts commit 09025dc8ef3e20403ad2a925287e6b418851f778.

There is no need to set ASM_ARCH unless assembly sources actually
are built. ASM_ARCH is no longer used for setting unrelated CFLAGS.
Merge pull request #812 from mstorsjo/remove-manual-win32-defines

Don't manually define WIN32/WIN64