[RISCV] Simplify fixup kinds that force relocations
For RELA targets, fixup kinds that force relocations (GOT, TLS, ALIGN,
RELAX, etc) can bypass `applyFixup` and be encoded as
`FirstRelocationKind+i`, as seen in LoongArch. This patch removes
redundant fixup kinds and adopts the `FirstRelocationKind+i` encoding.
The `llvm-mc -show-encoding` output no longer displays descriptive fixup
names, as this information is removed from
`RISCVAsmBackend::getFixupKindInfo`. While a backend hook could be added
to call `llvm::object::getELFRelocationTypeName`, it's unnecessary since
the relocation in `-filetype=obj` output is what truly matters.
Pull Request: https://github.com/llvm/llvm-project/pull/136088
