[llvm] [AArch64] Rename VariantKind to Specifier (PR #132595)

[Oliver Stannard via llvm-commits]

https://github.com/ostannard approved this pull request.

LGTM, assuming you update `test/MC/AArch64/coff-relocations-diags.s` which is failing in the pre-commits.

https://github.com/llvm/llvm-project/pull/132595