[llvm-branch-commits] Move relocation specifier constants to AArch64:: (PR #144633)

[Fangrui Song via llvm-branch-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/144633

Rename these relocation specifier constants, aligning with the naming
convention used by other targets (`S_` instead of `VK_`).

* ELF/COFF: AArch64MCExpr::VK_ => AArch64::S_ (VK_ABS/VK_PAGE_ABS are
  also used by Mach-O as a hack)
* Mach-O: AArch64MCExpr::M_ => AArch64::S_MACHO_
* shared: AArch64MCExpr::None => AArch64::S_None

Apologies for the churn following the recent rename in #132595. This
change ensures consistency after introducing MCSpecifierExpr to replace
MCTargetSpecifier subclasses.