Consolidate ABI parsing logic in TargetParser where
computeDefaultTargetABI is defined, instead of splitting it into the
backend. We need the full ABI information computable in
RuntimeLibcallsInfo

Prepare for removing ARMMCExpr. Adopt the new naming convention (S_
instead of VK_; the relocation specifier was previously named
`VariantKind`)) used by most other targets.

Make ARMMCAsmInfo.h include ARMMCExpr.h and change .cpp files to include
ARMMCAsmInfo.h. We will eventually remove ARMMCExpr.h.

Similar to previous migration done for other targets (PowerPC, X86,
etc).

Note: ARMELFObjectWriter::needsRelocateWithSymbol is conservative and
already includes most specifiers.

Migrate away from the deprecated MCSymbolRefExpr::VariantKind.
The name "Specifier" is utilized in a few *MCExpr.

> "Relocation specifier" is clear, aligns with Arm and IBM AIX's documentation, and fits the assembler's role seamlessly.

R_ARM_GOT_PREL is equivalent to GOTPCREL on other architectures, so we
can use it for indirect symbol replacement the same way. This is the
equivalent of https://github.com/llvm/llvm-project/pull/67754 for x86_64
and https://github.com/llvm/llvm-project/pull/78003 for AArch64 and
64-bit RISC-V.

Identified with readability-const-return-type.

When compiling for the RWPI relocation model the debug information is wrong:

* the debug location is described as { DW_OP_addr Var }
  instead of { DW_OP_constNu Var DW_OP_bregX 0 DW_OP_plus }
* the relocation type is R_ARM_ABS32 instead of R_ARM_SBREL32

Differential Revision: https://reviews.llvm.org/D111404

This change introduces support for zero flag ELF section groups to LLVM.
LLVM already supports COMDAT sections, which in ELF are a special type
of ELF section groups. These are generally useful to enable linker GC
where you want a group of sections to always travel together, that is to
be either retained or discarded as a whole, but without the COMDAT
semantics. Other ELF assemblers already support zero flag ELF section
groups and this change helps us reach feature parity.

Differential Revision: https://reviews.llvm.org/D95851