Add LoongArch64 support

[Chris-wh233]

The LoongArch architecture (LoongArch) is an Instruction Set Architecture (ISA) that has a RISC style.
Documentations:
ISA:
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html
ABI:
https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html
More docs can be found at:
https://loongson.github.io/LoongArch-Documentation/README-EN.html

Tested natively on a LoongArch64 machine.

LSX

Build flags: -mlsx
Result:
[doctest] test cases: 409 | 409 passed | 0 failed | 0 skipped
[doctest] assertions: 5976 | 5976 passed | 0 failed
[doctest] Status: SUCCESS!

LASX

Build flags: -mlasx
[doctest] test cases: 409 | 409 passed | 0 failed | 0 skipped
[doctest] assertions: 7912 | 7912 passed | 0 failed
[doctest] Status: SUCCESS!

[serge-sans-paille]

Thanks for the PR. Before going forward: if no CI is setup for this, we cannot maintain that contribution. Can you make the extra step?

Once CI is setup, I'll have a look at the actual implementation ;-)