rs6000: Support ELFv2 sibcall for indirect calls [PR96787]

rs6000: Support ELFv2 sibcall for indirect calls [PR96787]

Prior to P10, ELFv2 hasn't implemented nonlocal sibcalls.  Now that we do,
we need to be sure that r12 is set up prior to such a call.

2020-08-27  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	PR target/96787
	* config/rs6000/rs6000.c (rs6000_sibcall_aix): Support
	indirect call for ELFv2.

gcc/testsuite/

	PR target/96787
	* gcc.target/powerpc/pr96787-1.c: New.
	* gcc.target/powerpc/pr96787-2.c: New.