[BACKPORT] Disable generating load/store vector pairs for block copies.

[BACKPORT] Disable generating load/store vector pairs for block copies.

Testing has found that using load and store vector pair for block copies
can result in a slow down on power10.  This patch disables using the
vector pair instructions for block copies if we are tuning for power10.

2022-06-11   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000.cc (rs6000_option_override_internal): Do
	not generate block copies with vector pair instructions if we are
	tuning for power10.  Back port from master branch.