[llvm-objdump] Remove Generic Options group from help text output

Looking at other tools using tablegen for help output, general options
like --help are not separated from other options. This change removes
the "Generic Options" option group so the options are listed together.
the macho specific option group is left unaffected.

The test help.test was modified to reflect this change.

Differential Revision: https://reviews.llvm.org/D101652