Add LoongArch support

[Xiaotian Wu]

LoongArch is a new Loongson 3A5000 CPU instruction set, you can read 
documents[1] or visit the development community[2] to get more information.

[1]: https://loongson.github.io/LoongArch-Documentation/README-EN.html
[2]: https://github.com/loongson

This patch series will add the basic support for Loongarch architecture, it can 
compile on LoongArch and display the GRUB UI.

Please review the patches, thank you.