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 8 seconds (queued for 1 minute and 42 seconds)
latest
Status Job ID Name Coverage
  Build
passed #11720
docker
autoconf

00:04:35

passed #11721
docker
cmake

00:02:08

passed #11722
docker
meson

00:02:31

 
  Test
passed #11723
docker
whitespace

00:00:05