This patch adds Firefox cross-compilation from x86_64 Ubuntu 20.04 to
arm64 Ubuntu 20.04.
This patch also fixes packaging logic for Firefox to use a STRIP
command from the aarch64 toolchain.
References #7723
Apple M1 builders are now updated to the BigSur 11.6.
This patch:
- changes build assertions to validate 11.6 operating system
- changes build archive name to have major version only
- updates `//src/utils/registry.ts` to a new archive name for M1
This affects only M1 builds for Firefox, Firefox-Beta & WebKit.
We should roll these browsers at least once before 1.16 release.
