restore aarch64 JIT

Trying to keep most of the structural parts shared between x86_64 and
aarch64.  Not sure if this will stay factored like this long-term, but
the last version felt like there was a bit too much redundancy, and I
don't want to write things like register management more often than have
to.

Change-Id: Ieeb21f433715a730c41c85d657c5b33fa4702696
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228608
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed