MFC r243164 (by adrian):

I'm not sure why ah_desc.h was required here, but it doesn't _need_
to be. So, just toss it.

There's no options or ah_desc fields in here.

Whilst I'm here, fix up the #ifdef and #define to mach.
MFC r260020:

For sys/dev/drm2/radeon, only use -fms-extensions with gcc.  This flag
is only to stop gcc complaining about anonymous unions, which clang does
not do.  For clang 3.4 however, -fms-extensions enables the Microsoft
__wchar_t type, which clashes with our own types.h.