Add Loongarch64 support

[zhaixiaojuan]

Loongarch is a new RISC architecture.
Loongarch doc: https://github.com/loongson-community/LoongArch-Documentation/

[JohnTitor]

I think that isn't an official target of Rust, right? If so, for maintenance reasons, I'd recommend forking and patching yourself.

[zhaixiaojuan]

Thank you for your suggestion.
Currently, locally, our team has supported the code of the loongarch64 architecture in rust, libc and etc..., and the build has passed and the relevant tests have been run. The related patches are currently being sorted out. Later plans to submit rust, libc and other codes to the upstream community.

[JohnTitor]

Thanks for clarifying, I'd like to hold off this PR until PRs are sent to rustc and libc repos. I'm happy to merge this then :)

[heiher]

@JohnTitor The loongarch64-unknown-linux-gnu was added as a tier 3 target, could you review this patch again please?

[JohnTitor]

Thanks for the ping and contribution!