arc/gas: Accept, but ignore, dummy arguments.

arc/gas: Accept, but ignore, dummy arguments.

There's a set of legacy command line arguments that the arc assembler
still accepts, however, these arguments not longer have any effect on
the assembler.

Currently we return false from md_parse_option for all of these
arguments, with the result that the assembler terminates with an error
message.

We should return true indicating that the argument has been accepted,
even though we ignore it.

gas/ChangeLog:

	* config/tc-arc.c (md_parse_option): Return 1 in order to accept
	dummy arguments.