[LoongArch 1/6] Add triples loongarch{32,64} for the upcoming LoongArch target

This is the first patch to incrementally add an MC layer for LoongArch to LLVM.
This patch also adds unit testcases for these new triples.

RFC for adding this new backend:
https://lists.llvm.org/pipermail/llvm-dev/2021-December/154371.html

Differential revision: https://reviews.llvm.org/D115857