[PATCH][PR96791] disable POImode ld/st for memcpy

[PATCH][PR96791] disable POImode ld/st for memcpy

This is a (hopefully temporary) fix to PR96791. This will make
the default be -mno-block-ops-vector-pair even on power10, so we will
not hit the issue of DSE trying to truncate a POImode register. I am
still concerned it will be possible to hit this because the MMA builtins
will also generate POImode stores, but I think any example of that will
be somewhat more contrived.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (rs6000_option_override_internal):
	Change default.