[llvm-objdump] Use "--" for long options in --help text

Single dash for these options is not recognised.

Changes found by running this on the --help output
and the user guide:
grep -e ' -[a-zA-Z]\{2,\}'

The user guide was updated in https://reviews.llvm.org/D92305
so no change there.

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D92310