Reject -fno-multiflags [PR114314]

When -fmultiflags option support was added in r13-3693-g6b1a2474f9e422,
it accidently allowed -fno-multiflags which then would pass on to cc1.
This fixes that oversight.

Committed as obvious after bootstrap/test on x86_64-linux-gnu.

gcc/ChangeLog:

	PR driver/114314
	* common.opt (fmultiflags): Add RejectNegative.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>