Replacing !481 (closed), with the following changes:
- Rebased on
main(fixing conflicts mostly related to base-browser).
- Refactored the run-time SIMD check to actually check for ARM SIMD.
I haven't actually tried to build this yet, but fixing the merge conflicts should accelerate review, so I didn't want to make everyone wait for me to run an RBM build. If I run into build issues I'll submit a v4 MR with fixes.