[MC] [AArch64] Correctly resolve ":abs_g1:3" etc.

We have to treat constructs like this as if they were "symbolic", to use
the correct codepath to resolve them.  This mostly only affects movz
etc. because the other uses of classifySymbolRef conservatively treat
everything that isn't a constant as if it were a symbol.

Differential Revision: https://reviews.llvm.org/D55906

llvm-svn: 349800