This reduces the build time from 69 s to 30 s, reduces the size of
the built wels.lib from 30 MB to 3.9 MB, and reduces the number of
warnings when building wels.lib.
The architecture handling is now decoupled from the main
platform file. Now you can build for Windows Phone for x86 (for
the emulator) by "make OS=msvc-wp ARCH=x86".