rs6000: Builtins for doubleword compare should be in [power8-vector] (PR103625)

2021-12-13  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	PR target/103625
	* config/rs6000/rs6000-builtin-new.def (__builtin_altivec_vcmpequd):
	Move to power8-vector stanza.
	(__builtin_altivec_vcmpequd_p): Likewise.
	(__builtin_altivec_vcmpgtsd): Likewise.
	(__builtin_altivec_vcmpgtsd_p): Likewise.
	(__builtin_altivec_vcmpgtud): Likewise.
	(__builtin_altivec_vcmpgtud_p): Likewise.