gas: Handle bad -gdwarf options, just like bad --gdwarf options.

gas: Handle bad -gdwarf options, just like bad --gdwarf options.

parse_args uses getopt_long_only so it can handle long options both
with double and single dash. But this means that some single dash
options like -gdwarf-1 don't generate an error (unlike --gdwarf-1).

This is especially confusing since there is also --gdwarf2, but no
--gdwarf4 (it is --gdwarf-4). When giving -gdwarf4 the option is
silently interpreted as -g (which set dwarf_version to 2). This causes
some confusion for people who don't expect this and suddenly get
DWARF2 instead of DWARF4 as they might expect.

So make it so that the -gdwarf<unknown> creates an error, just like
--gdwarf<unknown> would.