RISC-V clang trunk generates opcodes that objdump doesn't understand

From a PR that ought to have fgixed it (#7332) -
_Originally posted by @mattgodbolt in https://github.com/compiler-explorer/compiler-explorer/issues/7332#issuecomment-2625686165_

Start by reading issue #7332 and the original linked comment, then reproduce the RISC-V clang trunk output and compare it with what objdump accepts. The issue names no files, tests, or entry points; done would require identifying the incompatibility and confirming a fix or clear resolution.

Written by the indexing model from the issue text.