Add -mdejagnu-cpu=power7 to dg-options for pr97142.c

To match the tests expectations for toolchains
configured to default to not so capable cpus.

2021-12-17  Olivier Hainque  <hainque@adacore.com>

gcc/testsuite/
	* gcc.target/powerpc/pr97142.c: Add -mdejagnu-cpu=power7
	to the dg-options.