Define SK_SUPPORT_LEGACY_X86_BLITS
Define SK_SUPPORT_LEGACY_X86_BLITS

This guards against changes I propose making to our various Src and SrcOver modes:
  - add SSE 4.1 and AVX2 implementations
  - fix a couple small precision issues
See https://codereview.chromium.org/1526883004/ for a start.

Those precision fixes will cause many minor layout test diffs.

BUG=

Review URL: https://codereview.chromium.org/1523363002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365338}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk