jumper, revert to generating .S files

jumper, revert to generating .S files

I went with the unified-in-one-.cpp approach mostly to make it easy to
roll out SkJumper.  I no longer see any difficultly rolling out the
assembly files, and it's possible the unified .cpp approach just makes
things harder.

Let's see if it's any easier to get Chrome's official build to work with
normal assembly files.  It's not going to be a problem to roll out.

This is a partial revert of https://skia-review.googlesource.com/c/9336.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Win2k8-MSVC-GCE-CPU-AVX2-x86_64-Debug,Test-Mac-Clang-MacMini6.2-CPU-AVX-x86_64-Debug,Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug,Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug

Change-Id: Idfdbd2d322452b44bc0adaf6dc299cc7649bc51e
Reviewed-on: https://skia-review.googlesource.com/10561
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>