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.

gcc/testsuite/gcc.target/powerpc/pr97142.c

 /* { dg-do compile } */
-/* { dg-options "-Ofast" } */
+/* { dg-options "-Ofast -mdejagnu-cpu=power7" } */
 
 #include <math.h>