Disallow -gno-dwarf, gno-dwarf-N, -gno-gdb and -gno-vms

The following adds RejectNegative to the gdwarf, gdwarf-, ggdb and gvms
options since the current behavior is to treat the negative variant
the same as the positive variant.  In particular -ggdb -gno-gdb
do not cancel, and plain -gno-dwarf will enable (dwarf!) debug output.

Rejecting the negative forms avoids interpreting sensible behavior
to combinations of options like -gdwarf-5 -gno-dwarf-3 and sticks to
the behavior that later -g options simply override earlier ones and
the only negative form is -g0.

	* common.opt (gdwarf): Add RejectNegative.
	(gdwarf-): Likewise.
	(ggdb): Likewise.
	(gvms): Likewise.