S/390: Fix rounding for _Decimal128 to _Decimal32 conversion

S/390: Fix rounding for _Decimal128 to _Decimal32 conversion

We do not have a direct conversion instruction from 128 bit DFP to 32
bit DFP so this needs to be done in two steps.  The first needs to be
done with the "prepare for shorter precision rounding mode" in order
to produce a correct result.

gcc/ChangeLog:

2016-03-11  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/s390/s390.md ("trunctddd2"): Turn former define_insn into
	define_expand.
	("*trunctddd2"): New pattern definition.
	("trunctdsd2"): Set prep_for_short_prec rounding mode for the
	TD->DD truncation.

gcc/testsuite/ChangeLog:

2016-03-11  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* gcc.target/s390/dfp-1.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@234134 138bc75d-0d04-0410-961f-82ee72b054a4