[LoongArch] Implement handling of triple-implied ABIs

According to the [[ https://loongson.github.io/LoongArch-Documentation/LoongArch-toolchain-conventions-EN.html | LoongArch Toolchain Conventions ]]
it is possible to specify the ABI modifier (the "D" part of "LP64D")
via the environment field in the target triple. This is needed for
proper support for Debian-style multiarch tuples as well, so add triple
awareness to `LoongArchSubtarget` via addition of
`LoongArchABI::computeTargetABI`. Let the explicit `--target-abi`
argument intuitively take precedence over the triple-implied ABI.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D142685