[MC] [AArch64] Support resolving fixups for abs_g0 etc.

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 opcode).

Differential Revision: https://reviews.llvm.org/D55896

llvm-svn: 349799