add #defines to limit SkCpu

I just noticed we don't really have any CPU test bots that have less
than AVX anymore.  I'd like to make sure we're testing each mode of
SkJumper at least, so I've added flags to let us limit to SSE2 or
SSE4.1, the modes currently missing coverage.

Add the bots to test these modes too.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Release-SK_CPU_LIMIT_SSE2,Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Release-SK_CPU_LIMIT_SSE41

Change-Id: I7c2b061332e7f037538488260583076c34ae7b1e
Reviewed-on: https://skia-review.googlesource.com/14405
Reviewed-by: Eric Boren <borenet@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed