Remove a duplicate -MD declaration
Move all x86 vs arm handling to msvc-common
Rename the msvc-arm platform file to msvc-wp

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".
Merge pull request #673 from mstorsjo/msvc-common-arch

Move all arch handling in msvc platform files into msvc-common