Comment out gnu specific stack notes on apple and armasm

Libav itself doesn't output these unless targeting ELF (where
they don't need to be commented out), but other projects (e.g.
OpenH264) have them unconditionally in the source.
Strip out more MSVC C compilation flags that don't apply when assembling

This is for use with OpenH264, which specifies more MSVC specific
compiler flags than libav does.
Add an option for forcing arm mode source to be assembled as thumb

This does some rudimentary conversions of instructions that aren't
available in thumb mode.

This allows building OpenH264 for Windows Phone 8 (and Windows RT),
which only supports thumb mode, and the OpenH264 arm assembly is
hardcoded for arm mode.

This is similar to how libvpx supports assembling in thumb mode - the
source itself is arm mode only, but a perl script can replace
instruction combinations that aren't supported in thumb mode. This
is a small subset of those conversions.