[Clang][Driver] Handle LoongArch multiarch tuples

This follows v1.00 of the [[ https://loongson.github.io/LoongArch-Documentation/LoongArch-toolchain-conventions-EN.html | LoongArch Toolchain Conventions ]],
but notably with [[ https://github.com/loongson/LoongArch-Documentation/pull/80 | this patch ]]
applied (a proper version bump to v2.00 was not done, so it is
indistinguishable from the "original" but now incompatible v1.00
otherwise).

Only `loongarch64` is implemented in `Linux::getMultiarchTriple`
because support for LA32 and ILP32* ABIs are incomplete right now.

The Debian sysroot layout is based on Han Gao's recent porting effort,
specifically the ghcr.io/rabenda/beige:loong64-v23-preview-20230330
container image.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D142688