rs6000: Support more SSE4 "cmp", "mul", "pack" intrinsics

Function signatures and decorations match gcc/config/i386/smmintrin.h.

Also, copy tests for:
- _mm_cmpeq_epi64
- _mm_mullo_epi32, _mm_mul_epi32
- _mm_packus_epi32
- _mm_cmpgt_epi64 (SSE4.2)

from gcc/testsuite/gcc.target/i386.

2021-10-11  Paul A. Clarke  <pc@us.ibm.com>

gcc
	* config/rs6000/smmintrin.h (_mm_cmpeq_epi64, _mm_cmpgt_epi64,
	_mm_mullo_epi32, _mm_mul_epi32, _mm_packus_epi32): New.
	* config/rs6000/nmmintrin.h: Copy from i386, tweak to suit.

gcc/testsuite
	* gcc.target/powerpc/pr78102.c: Copy from gcc.target/i386,
	adjust dg directives to suit.
	* gcc.target/powerpc/sse4_1-packusdw.c: Same.
	* gcc.target/powerpc/sse4_1-pcmpeqq.c: Same.
	* gcc.target/powerpc/sse4_1-pmuldq.c: Same.
	* gcc.target/powerpc/sse4_1-pmulld.c: Same.
	* gcc.target/powerpc/sse4_2-pcmpgtq.c: Same.
	* gcc.target/powerpc/sse4_2-check.h: Copy from gcc.target/i386,
	tweak to suit.