Use the GCC specific codepath for Clang in MSVC mode too.
This matches the condition used in a number of other places.

MSVC doesn't have a way to signal potential aliasing, while GCC
(and Clang) can use __attribute__((may_alias)) for this purpose.

When building with Clang in MSVC mode, __GNUC__ isn't defined but
_MSC_VER is as Clang primarily impersonates MSVC - but even then it
does support the GCC style attributes.

The GCC specific codepath uses av_alias, which expands to
the may_alias attribute if supported. The MSVC specific codepath
doesn't use av_alias so far (as MSVC doesn't support any
corresponding attribute).

This fixes a couple HEVC decoder tests when built with Clang 14 or
newer in MSVC mode (with issues observed on all of x86_64, armv7
and aarch64).
Signed-off-by: Martin Storsjö <martin@martin.st>