Shift ELF `@plt` and `@gotpcrel` references in data directives, as well as
Mach-O `@specifier` notations, to use `AArch64MCExpr::Specifier` constants.
This is a follow-up to #132595. COFF-specific specifiers are not moved
yet.
In addition, partition @-specifiers into COFF, ELF, and Mach-O, so that
mix-and-match is rejected at parse time.
ELF and Mach-O specifiers are distinct, with `None` being the only
shared value. For Mach-O-specific specifiers, we adopt the `M_xxx` naming
convention.
Pull Request: https://github.com/llvm/llvm-project/pull/133214
71884b63a4