remove vfpv4 requirement for SkJumper on ARMv7

VFPv4 gives us two interesting features:
  - FMA
  - f16<->f32 conversions

Even without FMAs, NEON still has non-fused MLA instructions.  We don't
really care about the fusedness of those mul-adds, so losing FMA here is
kind of no big deal.

We already maintain portable code to do f16<->f32 conversions, so it's
not much of a maintanence hit to use that instead of the native
instructions.  To my knowledge software F16 rendering is not a
performance critical mode of operation for any of our users.

This drops our minimum requirement to basically just having NEON.
Devices like the Nexus 7 2012 will now take SkJumper fast paths
instead of portable code.  (Though actually, we've only ever
required NEON for _lowp... only the float code also needed vfpv4).

The main file to look at here is actually SkJumper_vectors.h,
where you will see all the substantive changes.  The rest just
kind of tears down most of the old complexity, add adds ABI
to put just a little of it back.  :)

Change-Id: Ia9237117698729c91e5fa51126baf80748093bf4
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/83521
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>

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