Add support for Linux aarch64

[jasonstack]

• add gcc target for linux-aarch64

• for linux, build only current architecture

• boring SSL build command is the same as linux x86 platform

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[wendigo]

It'd be nice to get this in :)

[bruestel]

Thank you for this pull request—it works perfectly, and I think it would be fantastic to see it officially merged!

The main challenge, however, is the lack of ARM-based runners on GitHub Actions, which might make it harder for others to test this directly. One potential workaround could be using QEMU to emulate an ARM environment.

For my small hobby project, I set up a self-hosted ARM-based runner, and everything worked flawlessly. Here is the link to the action build for reference: Action Build Link.

[prbprbprb]

Drive by comment: While this works fine for local builds on ARM devices so scratches an itch for people, it's not ideal for releases where cross-compiling and using Clang would be preferable so only one release build machine is needed.

Also #1386 and #1388 move the goalposts a bit.