use -mavx clang style options to enable SIMD for clang-cl

[wangyoucao577]

• Only use /arch:AVX MSVC style options to enable SIMD when strictly MSVC cl;
• Use -mavx gcc/clang style options to enable SIMD when possible, for example clang-cl installed in MSVC;
• Enable C11 for clang-cl;
• Enable cpuinfo for clang-cl;

Closes https://github.com/xiph/opus/issues/256

[xnorpx]

@wangyoucao577 can you add the same check for all other compiler flags regarding clang-cl frontend. (fast-math etc)

[wangyoucao577]

@wangyoucao577 can you add the same check for all other compiler flags regarding clang-cl frontend. (fast-math etc)

I checked all MSVC related options in current cmake, unfortunately -ffast-math can't work on clang-cl, it must be /fp:fast. Supported options(c11, cpuinfo) has been updated.

[jmvalin]

What's the status of this PR?

[abique]

Is it the correct fix to enable avx everywhere? If you do that then it'll allow the compiler to use avx regardless of the intrinsic right?

[xnorpx]

Master branch is deleted and all pr's against master is closed. If this change is still relevant please reopen and repoint your PR to main branch.