to align with targets that have made the transition.

* Use MCAsmInfo::printSpecifierExpr instead of MCExpr::print.
* Replace PPCMCExpr with MCSpecifierExpr.

Prepare for removing PPCMCExpr. Adopt the newer naming convention with
AMDGPU/WebAssembly/VE/M68k.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

In addition, rename *MCExpr::getKind, which confusingly shadows the base class getKind.

Use the more appropriate term "relocation specifier" and avoid the
variable name `Kind`, which conflicts with MCExpr and FixupKind.

Follow the X86 and Mips renaming.

> "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’s usage, and fits the assembler's role seamlessly.

In addition, rename *MCExpr::getKind, which confusingly shadows the base class getKind.

Follow-up to d6fbffa23c84e622735b3e880fd800985c1c0072 . This commit
updates all call sites and removes the argument from the function.

The signedness of the @l result is dependent on the instruction operand
(gas behavior).
E.g. in `addis 3,3,65535@l`, 65535@l is signed. Unfortunately
we don't have the information.

bfef1dd694d4c646f79521fa3258bbb2398a990c (2014) checked `Fixup`,
which was unnecessary and mislead https://reviews.llvm.org/D115419
to make the code more complex.

In PPCMCExpr::evaluateAsRelocatableImpl we don't need to validate the
result. Just continue and rely on the validation in ELFObjectWriter.