PowerPC: Make -mcpu=future enable -mblock-ops-vector-pair.

PowerPC: Make -mcpu=future enable -mblock-ops-vector-pair.

This patch enables generating load and store vector pair instructions when
doing certain memory copy operations when -mcpu=future is used.  In doing tests
on power10, it was determined that using these instructions were problematical
in a few cases, so we disabled generating them by default.  This patch
re-enabled generating these instructions if -mcpu=future is used.

The patches have been tested on the following platforms.  I added the patches
for PR target/107299 that I submitted on November 2nd before doing the builds so
that GCC would build on systems using IEEE 128-bit long double.
    *	https://gcc.gnu.org/pipermail/gcc-patches/2022-November/604834.html

There were no regressions with doing bootstrap builds and running the regression
tests:

    1)	Power10 LE using --with-cpu=power10 --with-long-double-format=ieee;
    2)	Power10 LE using --with-cpu=power10 --with-long-double-format=ibm;
    3)	Power9 LE using --with-cpu=power9 --with-long-double-format=ibm; and
    4)	Power8 BE using --with-cpu=power8 (both 32-bit & 64-bit tested).

Note, I will be on vacation from Tuesday February 7th through Tuesday February
14th.

Can I check this patch into the GCC 13 master branch?

2023-02-17   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000-cpus.def (ISA_FUTURE_MASKS): Add
	-mblock-ops-vector-pair.
	(POWERPC_MASKS): Likewise.