[llvm] [llvm][docs] Update list of working Linux host platforms (PR #149503)

[Lu Weining via llvm-commits]

SixWeining wrote:

> @llvm/issue-subscribers-backend-loongarch , anyone able to confirm my assertion here?

Yes. Several Linux distributions have officially or unofficially supported LoongArch. For example, [Alpine](https://wiki.alpinelinux.org/wiki/Loongarch64)  and [Debian](https://wiki.debian.org/LoongArch).


https://github.com/llvm/llvm-project/pull/149503