Send patch to LLVM adding the muslabin32 target environment

[andrewrk]

musl supports the mipsn32 ABI but LLVM only has musl, musleabi, and musleabihf.

This C ABI is therefore unavailable in Zig even though musl provides it.

This should be an uncontroversial patch that I expect LLVM to accept easily.

https://github.com/llvm/llvm-project/blob/c23619b0c90056f9bd63f9b5d79caf5bf63618e8/llvm/include/llvm/ADT/Triple.h#L209-L211