[llvm-objdump] Add -mllvm (#75892)

When llvm-objdump switched from cl:: to OptTable
(https://reviews.llvm.org/D100433), we dropped support for LLVM cl::
options. Some LLVM_DEBUG in `llvm/lib/Target/$target/MCDisassembler/`
files might be useful. Add -mllvm to allow dumping the information.

```
# -debug is available in an LLVM_ENABLE_ASSERTIONS=on build
llvm-objdump -d -mllvm -debug a.o > /dev/null
```

Link:
https://discourse.llvm.org/t/how-to-enable-debug-logs-in-llvm-objdump/75758