rs6000: __builtin_mma_disassemble_acc() doesn't store elements correctly in LE mode

rs6000: __builtin_mma_disassemble_acc() doesn't store elements correctly in LE mode

PR96236 shows a problem where we don't correctly store our 512-bit accumulators
correctly in little-endian mode.  The patch below detects when we're doing a
little-endian memory access and stores to the correct memory locations.

2020-07-22  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/96236
	* config/rs6000/rs6000-call.c (rs6000_gimple_fold_mma_builtin): Handle
	little-endian memory ordering.

gcc/testsuite/
	PR target/96236
	* gcc.target/powerpc/mma-double-test.c: Update storing results for
	correct little-endian ordering.
	* gcc.target/powerpc/mma-single-test.c: Likewise.