Removes the custom code for VS2012 support in config.h,
and reduces diff with upstream config.h.
We still have many custom defines for ARM optimizations
which we probably don't use properly, since we don't
compile the included asm code, so a thorough review and
cleanup would be welcome.
Took the opportunity to undo the Godot changed made to the
opus source. The opus module should eventually be built in its
own environment to avoid polluting others with too many include
dirs and defines.
TODO: Fix the platform/ stuff for opus.
