[clang] [llvm] [MIPS] Add support for O64 Mips ABI (PR #223990)

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LunaTheFoxgirl wrote:

Just for reference, my experience with MIPS specifically is not that extensive, so besides local testing I'm not 100% sure on details in places.

My expertise is mostly in regard to SuperH. Hopefully I'm not too frustrating to work with here ^^;

https://github.com/llvm/llvm-project/pull/223990