gas: support --gsframe=no

Being able to explicitly disable SFrames on the command line is useful,
especially when looking at a gas that enables SFrames by default. The
binutils testsuite will benefit from this as there's testcases that don't
expect their presence.

In summary:
* Nothing is passed       => no SFrames (no change from before)
* --gsframe is passed     => SFrames    (no change from before)
* --gsframe=yes is passed => SFrames    (previously rejected)
* --gsframe-no  is passed => no SFrames (previously rejected)

	PR gas/33125
	* gas/as.c (parse_args): Accept --gsframe=no, --gsframe=yes.