[AArch64] Rename VariantKind to Specifier
Follow the naming of most other backends.
> "Relocation modifier" suggests adjustments happen during the linker's relocation step rather than the assembler's expression evaluation.
> "Relocation specifier" is clear, aligns with Arm and IBM AIX's documentation, and fits the assembler's role seamlessly.
In addition, rename `AArch64MCExpr::Kind` and `getKind`, which confusingly shadow the base class `Kind` and `getKind`.
DarwinRefKind, which still uses MCSymbolRefExpr::VariantKind, is not renamed.
Pull Request: https://github.com/llvm/llvm-project/pull/132595
