testsuite: Another fixup for fixed-point/bitint-1.c test

testsuite: Another fixup for fixed-point/bitint-1.c test

Besides r16-3595, there's another bug in this test: with -std=c23 the
token _Sat isn't recognized as a keyword at all, thus an error massage
different from the expected will be outputted.

Fix it by using -std=gnu23 instead.

gcc/testsuite:

	* gcc.dg/fixed-point/bitint-1.c (dg-options): Use -std=gnu23
	instead of -std=c23.