Adding FMA and AVX2 config switches

Most modern x86 processors have FMA and AVX2, which OPUS could leverage
for better performance.

This commit replaces existing AVX flag to a new flag guarding all AVX,
FMA, and AVX2; mainly because:
1. AVX-optimized FP code usually comes with opportunities for FMA;
2. CPUs support FMA but not AVX2 are rare (AMD Bulldozer Gen 2 and 3);
3. No CPU supports AVX2 but not FMA;
4. MSVC's /arch:AVX2 controls both FMA and AVX2.

Signed-off-by: Victor Ding <victording@google.com>
4 jobs for avx2 in 5 minutes and 3 seconds (queued for 2 minutes and 24 seconds)
Status Name Job ID Coverage
  Build
passed autoconf #11961
docker

00:04:38

passed cmake #11962
docker

00:02:09

passed meson #11963
docker

00:02:29

 
  Test
passed whitespace #11964
docker

00:00:05