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.