[ADT][Triple] Add environment kinds for LoongArch GNU multiarch tuples

The canonical multiarch tuples for LoongArch are defined in [the
LoongArch toolchain conventions][1] document. As the musl port is still
WIP, only the GNU triples are added for now.

The spec mentions `loongarch64-linux-gnuf64`, which is functionally the
same as the existing `loongarch64-linux-gnu` triple, only with the
floating-point ABI part explicitly spelled out. Both forms are
supported, but normalization of one into another is not implemented in
this patch, to give the ecosystem some time to experiment and discuss.

[1]: https://loongson.github.io/LoongArch-Documentation/LoongArch-toolchain-conventions-EN.html

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D135751