rs6000: Generates permute index directly for little endian targets (PR100866)

rs6000: Generates permute index directly for little endian targets (PR100866)

2022-10-11  Haochen Gui <guihaoc@linux.ibm.com>

gcc/
	PR target/100866
	* config/rs6000/rs6000-call.cc (swap_endian_selector_for_mode):
	Generate permute index directly for little endian targets.
	* config/rs6000/vsx.md (revb_<mode>): Call vprem directly with
	corresponding permute indexes.

gcc/testsuite/
	PR target/100866
	* gcc.target/powerpc/pr100866-1.c: New.