rs6000: Add tests for SSE4.1 "floor" intrinsics

rs6000: Add tests for SSE4.1 "floor" intrinsics

Add the tests for _mm_floor_pd, _mm_floor_ps, _mm_floor_sd, _mm_floor_ss.
These are modelled after (and depend upon parts of) the tests for
_mm_ceil intrinsics, recently posted.

Copy a test for _mm_floor_sd from gcc/testsuite/gcc.target/i386.

2021-07-30  Paul A. Clarke  <pc@us.ibm.com>

gcc/testsuite
	* gcc.target/powerpc/sse4_1-floorpd.c: New.
	* gcc.target/powerpc/sse4_1-floorps.c: New.
	* gcc.target/powerpc/sse4_1-floorsd.c: New.
	* gcc.target/powerpc/sse4_1-floorss.c: New.
	* gcc.target/powerpc/sse4_1-roundpd-2.c: Copy from
	gcc/testsuite/gcc.target/i386 and adjust dg directives to suit.