Move CPU feature detection to its own file.

Move CPU feature detection to its own file.

   - Moves CPU feature detection to its own file.
   - Cleans up some redundant feature detection scattered around core/ and opts/.
   - Can now detect a few new CPU features:
       * F16C     -> Intel f16<->f32 instructions, added between AVX and AVX2
       * FMA      -> Intel FMA instructions, added at the same time as AVX2
       * VFP_FP16 -> ARM f16<->f32 instructions, quite common
       * NEON_FMA -> ARM FMA instructions, also quite common
       * SSE and SSE3... why not?

This new internal API makes it very cheap to do fine-grained runtime CPU
feature detection.  Redundant calls to SkCpu::Supports() should be eliminated
and it's hoistable out of loops.  It compiles away entirely when we have the
appropriate instructions available at compile time.

This means we can call it to guard even a little snippet of 1 or 2 instructions
right where needed and let inlining hoist the check (if any at all) up to
somewhere that doesn't hurt performance.  I've explained how I made this work
in the private section of the new header.

Once this lands and bakes a bit, I'll start following up with CLs to use it more
and to add a bunch of those little 1-2 instruction snippets we've been wanting,
e.g. cvtps2ph, cvtph2ps, ptest, pmulld, pmovzxbd, blendvps, pshufb, roundps
(for floor) on x86, and vcvt.f32.f16, vcvt.f16.f32 on ARM.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1890483002
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Committed: https://skia.googlesource.com/skia/+/872ea29357439f05b1f6995dd300fc054733e607

Review URL: https://codereview.chromium.org/1890483002