WinRT: fixed libvorbis and libvorbisfile build errors when building for ARM

The fix is a bit of a hack: it defines '_WIN32_WCE' via the C preprocessor,
which will end up preventing some of libvorbis' x86 assembly code
(in external/libvorbis-1.3.3/lib/os.h) from attempting to be used in WinRT + ARM
builds.  The fix was done this way to keep it limited to WinRT project files,
and out of builds for other platforms.  A broader fix may, at some point, be
better off placed in libvorbis' source code, however this seems to work for now.

Non-ARM or non-WinRT builds of libvorbis and libvorbisfile are unaffected by
this change.
Updated iOS deployment target to iOS 5.1
WinRT: added project files for Windows 8.1 and Windows Phone 8.1 targets
Made Win[Phone] 8.1 project file names more consistent with 8.0 counterparts

This renames the two, main pairs of SDL_mixer project files, for Windows 8.1
and Windows Phone 8.1, to have file names that are a bit more consistent with
their 8.0 counterparts.  I.e. "SDL_mixer-<platform>.<ext>"