Build the Tor and Mullvad Browsers for aarch64 Linux

Opening a new issue, separate from #12631 and #32355, since those also mention the 32-bit linux arm target which you may want to build in the future. For the record, by now I've ported the browser to 32-bit linux-arm too, but the builds are largely (but not completely) untested for lack of hardware.

My next MR will be filed against this bug.