rs6000: Add test for _mm_minpos_epu16

Copy the test for _mm_minpos_epu16 from
gcc/testsuite/gcc.target/i386/sse4_1-phminposuw.c, with
a few adjustments:

- Adjust the dejagnu directives for powerpc platform.
- Make the data not be monotonically increasing,
  such that some of the returned values are not
  always the first value (index 0).
- Create a list of input data testing various scenarios
  including more than one minimum value and different
  orders and indices of the minimum value.
- Fix a masking issue where the index was being truncated
  to 2 bits instead of 3 bits, which wasn't found because
  all of the returned indices were 0 with the original
  generated data.
- Support big-endian.

2021-08-03  Paul A. Clarke  <pc@us.ibm.com>

gcc/testsuite
	* gcc.target/powerpc/sse4_1-phminposuw.c: Copy from
	gcc/testsuite/gcc.target/i386, adjust dg directives to suit,
	make more robust.

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