rs6000: Generate _Decimal128 to _Decimal32 hardware conversion instructions

rs6000: Generate _Decimal128 to _Decimal32 hardware conversion instructions

We do not currently generate hardware conversion instructions when
converting from _Decimal128 to _Decimal32.  There is no one instruction
that does the conversion, so we currently call the __dpd_trunctdsd2
lib function to do the conversion for us.  However, there is a short
sequence of dfp hardware instructions that will do the conversion
correctly.

2020-07-17  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/92488
	* config/rs6000/dfp.md (trunctdsd2): New define_insn.
	* config/rs6000/rs6000.md (define_attr "isa"): Add p9.
	(define_attr "enabled"): Handle p9.

gcc/testsuite/
	PR target/92488
	* gcc.target/powerpc/convert-fp-128.c (bl, drsp, drdpq): Update counts.
	(__dpd_trunctdsd2): Make conditional on !hard_dfp.
	(__dpd_extendsddd2, __dpd_extendsdtd2, __dpd_truncddsd2,
	__dpd_extendddtd2, __dpd_trunctddd2): Use !hard_dfp.
	* gcc.target/powerpc/pr92488.c: New test.