libavutil: Hook up the rest of the gcc specific attributes to clang as well

libavutil: Hook up the rest of the gcc specific attributes to clang as well

Hook up all attributes that don't have a MSVC specific version at the
moment.

See f637046d313 for details.

These don't seem to be critical for building with clang in MSVC mode
though, and thus haven't been hooked up until now.

These seem to build fine with as old clang as 3.3 at least.
(clang 3.3 disguises itself as gcc 4.2 normally, so all of these
have been used for clang before, except for av_cold.)

The clang version numbers themselves are useless for detecting what
attributes are available, since Apple's clang builds use a completely
different versioning (presenting itself as e.g. clang 8.0 instead
of 3.8).

Signed-off-by: Martin Storsjö <martin@martin.st>