split up JUMPER define

Whether JUMPER is defined is starting to get a little overloaded:
   - are we compiling offline (defined) or as part of Skia (!defined)?
   - are we using Clang vector extensions (defined) or scalars (!defined)?

This splits JUMPER into these two separate concerns:
   - JUMPER_IS_OFFLINE
   - JUMPER_IS_SCALAR, JUMPER_IS_NEON, JUMPER_IS_AVX2, etc.

The upshot is that we'll now use Clang vector extensions when available
for our "portable" baseline.  On x86-64 and ARMv8 compiled by Clang,
we're guaranteed to pick up SSE2 and NEON respectively.  Our -Fast
bot should even get all the way to AVX2.

Another CL will do some refactoring in SkJumper to remove the redundant
copies of guaranteed vector code on x86-64 and ARMv8.  I didn't want to
do that here yet to demonstrate that there is zero effect on the .S
files from this CL.

Change-Id: Ib5e8f00b35e8721b2cc7180e294840ffaf9dddce
Reviewed-on: https://skia-review.googlesource.com/39500
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
6 files changed