rs6000/test: Add emulated gather test case

As verified, the emulated gather capability of vectorizer
(r12-2733) can help to speed up SPEC2017 510.parest_r on
Power8/9/10 by 5% ~ 9% with option sets Ofast unroll and
Ofast lto.

This patch is to add a test case similar to the one in i386
to add testing coverage for 510.parest_r hotspots.

btw, different from the one in i386, this uses unsigned int
as INDEXTYPE since the unpack support for unsigned int
(r12-3134) also matters for the hotspots vectorization.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/vect-gather-1.c: New test.