Use a variable symbol without any specifier instead of VK_WEAKREF.
Add code in ELFObjectWriter::executePostLayoutBinding to check
whether the target should be made an undefined weak symbol.

This change fixes several issues:

* Unreferenced `.weakref alias, target` no longer creates an undefined `target`.
* When `alias` is already defined, report an error instead of crashing.

.weakref is specific to ELF. llvm-ml has reused the VK_WEAKREF name for
a different concept. wasm incorrectly copied the ELF implementation.
Remove it.

Remove FK_PCRel_* kinds from the generic fixup list, as they are not
generic like FK_Data_*. In getRelocType, FK_PCRel_* can be replaced with
FK_Data_* by leveraging the IsPCRel argument. Their inclusion in the
generic kind list caused confusion for PowerPC, RISCV, and VE targets.

The X86/M68k uses can be implemented as target-specific fixups.

* Remove the MCAssembler * argument. Change subclasses to use MCAssembler *MCObjectWriter::Asm.
* Remove pure specifier and add an empty implementation
* Change MCFragment * to MCFragment &

Move wasm-specific members outside of MCSymbolRefExpr::VariantKind (a
legacy interface I am eliminating). Most changes are mechanic and
similar to what I've done for many ELF targets (e.g. X86 #132149)

Notes:

* `fixSymbolsInTLSFixups` is replaced with `setTLS` in
  `WebAssemblyWasmObjectWriter::getRelocType`, similar to what I've done
  for many ELF targets.
* `SymA->setUsedInGOT()` in `recordRelocation` is moved to
  `getRelocType`.

While here, rename "Modifier' to "Specifier":

> "Relocation modifier", though concise, suggests adjustments happen during the linker's relocation step rather than the assembler's expression evaluation. I landed on "relocation specifier" as the winner. It's clear, aligns with Arm and IBM’s usage, and fits the assembler's role seamlessly.

Pull Request: https://github.com/llvm/llvm-project/pull/133116

Commit 52eb11f925ddeba4e1b3840fd636ee87387f3ada temporarily introduced
getSymSpecifier to prepare for "MCValue: Replace MCSymbolRefExpr members
with MCSymbol" (d5893fc2a7e1191afdb4940469ec9371a319b114). The
refactoring is now complete.

We will replace the MCSymbolRefExpr member in MCValue with MCSymbol.
This change reduces dependence on MCSymbolRefExpr.