[llvm] [RISCV] Xqcilb: rename JUMP_PLT fixup and drop `@plt` parsing (PR #135507)

[Sam Elliott via llvm-commits]

lenary wrote:

It looks like we're going to keep disagreeing about whether the `@plt` is useful or misleading, but if there are weird parsing artefacts when there's an offset, then I'm happy to drop support for it for Xqcilb. 

I will update my docs to reflect that the `@plt` is not needed.

I think the fixup should not be renamed. It is aligned with the relocation name, and I think the correspondence is useful and valid. I haven't seen a clear reason to rename the relocation from this discussion, and continue to find the fact the relocation ends in `_PLT` to be useful.

I think that we no longer need `PseudoQCJumpSymbol`, `pseudo_qc_jump_symbol`, `PseudoQC_E_J`, and `PseudoQC_E_JAL`, all of which were just for the custom symbol parsing. If we accept a bare symbol, I think we just update `simm32_lsb0` to instead be `bare_simm32_lsb0` in `QCIRVInst48EJ`, and have it accept bare symbols.

https://github.com/llvm/llvm-project/pull/135507