Draft: Bug 32355 v3: Tor Browser for Linux/ARMv7 (x86_64 build arch)

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.

Closes #32355

/cc @boklm