rs6000: Support SSE4.1 "min" and "max" intrinsics

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

Also, copy tests for _mm_min_epi8, _mm_min_epu16, _mm_min_epi32,
_mm_min_epu32, _mm_max_epi8, _mm_max_epu16, _mm_max_epi32, _mm_max_epu32
from gcc/testsuite/gcc.target/i386.

sse4_1-pmaxsb.c and sse4_1-pminsb.c were modified from using
"char" types to "signed char" types, because the default is unsigned on
powerpc.

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

gcc
	* config/rs6000/smmintrin.h (_mm_min_epi8, _mm_min_epu16,
	_mm_min_epi32, _mm_min_epu32, _mm_max_epi8, _mm_max_epu16,
	_mm_max_epi32, _mm_max_epu32): New.

gcc/testsuite
	* gcc.target/powerpc/sse4_1-pmaxsb.c: Copy from gcc.target/i386.
	* gcc.target/powerpc/sse4_1-pmaxsd.c: Same.
	* gcc.target/powerpc/sse4_1-pmaxud.c: Same.
	* gcc.target/powerpc/sse4_1-pmaxuw.c: Same.
	* gcc.target/powerpc/sse4_1-pminsb.c: Same.
	* gcc.target/powerpc/sse4_1-pminsd.c: Same.
	* gcc.target/powerpc/sse4_1-pminud.c: Same.
	* gcc.target/powerpc/sse4_1-pminuw.c: Same.