Add support for LoongArch.

[liuxiang88]

Add support for LoongArch.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

[svaarala]

Thanks @liuxiang88, could you update AUTHORS.rst (see CONTRIBUTING.md)?

As a sidenote, I'd like to allow running testcases for LoongArch, can you recommend a physical device or virtualization environment (qemu)?

[liuxiang88]

Sorry，after a week of trying, I still can't provide a physical device yet.The following steps are to create the virtuallization environment for LA：

1. Install qemu. If can't, can make one.
2. Making LA rootfs .

LA-qemu-rootfs.md

[svaarala]

Thanks @liuxiang88!