[lld] [llvm] [RISCV] Add an instruction PrettyPrinter to llvm-objdump (PR #90093)

[Alex Bradbury via llvm-commits]

asb wrote:

> It looks like the current code handles 16 and 32 bit instructions.

I wondered about > 32-bit instructions too. By my reading, it will print the first 32 bits and then use `dumpBytes` for the rest. So we'll get output, though  may later want to agree what format we actually want for that case. Perhaps a TODO code comment that we may want to agree the desired format for 48-bit instruction hex when they're introduced.

LGTM, thanks Craig. I think a release note might be nice to have too, as it's user facing and also people may have objdump parsing scripts that might be affected.

https://github.com/llvm/llvm-project/pull/90093