MIPS64 LLVM backend

For fun and to learn more about GHC's LLVM code generator I've started to build a MIPS64 LLVM target.

MIPS registerised support was once removed (7b594a5d), but there seems to be some activity around MIPS64, especially if you look for loongson.

At least this would help Linux distribution projects, like Debian (https://wiki.debian.org/MIPSPort), to run their packaging pipelines much faster.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information