testsuite: Adjust rs6000-ldouble-2.c for switch to -std=gnu23 by default [PR117663]

testsuite: Adjust rs6000-ldouble-2.c for switch to -std=gnu23 by default [PR117663]

-std=gnu23/-std=c23 changes LDBL_EPSILON for IBM long double, see r13-3029 and
https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602738.html
for details.

That change even had a note:
"and when we move to a C2x
default, gcc.target/powerpc/rs6000-ldouble-2.c will need an
appropriate option added to keep using an older language version"

The following patch just implements it to fix rs6000-ldouble-2.c regression.

2024-12-02  Jakub Jelinek  <jakub@redhat.com>

	PR testsuite/117663
	* gcc.target/powerpc/rs6000-ldouble-2.c: Add -std=gnu17 to dg-options.