rs6000: Remove ISA_3_0_MASKS_IEEE and check P9_VECTOR instead

When working to get rid of mask bit OPTION_MASK_P8_VECTOR,
I noticed that the check on ISA_3_0_MASKS_IEEE is actually
to check TARGET_P9_VECTOR, since we check all three mask
bits together and p9 vector guarantees p8 vector and vsx
should be enabled.  So this patch is to adjust this first
as preparatory patch for the following patch to change
all uses of OPTION_MASK_P8_VECTOR and TARGET_P8_VECTOR.

gcc/ChangeLog:

	* config/rs6000/rs6000-cpus.def (ISA_3_0_MASKS_IEEE): Remove.
	* config/rs6000/rs6000.cc (rs6000_option_override_internal): Replace
	ISA_3_0_MASKS_IEEE check with TARGET_P9_VECTOR.