SkJumper: start on asm

Will follow up with Linux, then Android aarch64 and armv7, then iOS, then Windows.

I took some opportunities to refactor.

CQ_INCLUDE_trybots=skia.primary:Test-Mac-Clang-MacMini6.2-CPU-AVX-x86_64-Debug,Perf-Mac-Clang-MacMini6.2-CPU-AVX-x86_64-Debug

Change-Id: Ifcf1edabdfe5df0a91bd089f09523aba95cdf5ef
Reviewed-on: https://skia-review.googlesource.com/8611
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
6 files changed