Port SkXfermode opts to SkOpts.h

Port SkXfermode opts to SkOpts.h

Renames Sk4pxXfermode.h to SkXfermode_opts.h,
and refactors it a tiny bit internally.

This moves xfermode optimization from being "compile-time everywhere but NEON"
to simply "runtime everywhere".  I don't anticipate any effect on perf or
correctness.

BUG=skia:4117

Review URL: https://codereview.chromium.org/1264543006