Set ASM_ARCH in all cases to fix Win64 Debug build


ASM_ARCH affects how the CFLAGS are set in platform-msvc-common.mk, but it is not set in platform-x86-common.mk unless it is a Release build.  This is causing the Win64 Debug build to fail i.e. "make OS=msvc BUILDTYPE=Debug ENABLE64BIT=Yes"

The simplest solution is to set ASM_ARCH in all cases in platform-x86-common.mk.  Alternatively we could change platform-msvc-common.mk to change the CLFAGS outside of the ASM_ARCH check.