This requires a bit more code than other fixups, to distingush between abs_g0/abs_g1/etc. Actually, I think some of the other fixups are missing some checks, but I won't try to address that here.
I haven't seen any real-world code that uses a construct like this, but it clearly should work, and we're considering using it in the implementation of localescape/localrecover on Windows (see https://reviews.llvm.org/D53540). I've verified that binutils produces the same code as llvm-mc for the testcase.
This currently doesn't include support for the *_s variants (that requires a bit more work to set the instruction opcode).