rs6000/testsuite: Use the int128 selector where needed

rs6000/testsuite: Use the int128 selector where needed

Tests that use the __int128 type need to use the int128 selector.

2020-05-15  Segher Boessenkool  <segher@kernel.crashing.org>

gcc/testsuite/
	* gcc.target/powerpc/vec-gnb-0.c: Use int128 effective target.
	* gcc.target/powerpc/vec-gnb-1.c: Ditto.
	* gcc.target/powerpc/vec-gnb-2.c: Ditto.
	* gcc.target/powerpc/vec-ternarylogic-8.c: Ditto.
	* gcc.target/powerpc/vec-ternarylogic-9.c: Ditto.
	* gcc.target/powerpc/vec-ternarylogic-10.c: Ditto.