Bug 472875 - s390x regtest: Rework dfp-1

Bug 472875 - s390x regtest: Rework dfp-1

The dfp-1 test case has two main issues:

* It doesn't initialize the condition code before executing the DFP
  instructions for "multiply" and "divide", but asserts a certain value
  afterwards.  (See Bug 472875.)

* It doesn't compile with Clang.  (See Bug 465782.)

Fix these with a rework.  The first issue can be fixed by initializing the
condition code before executing the DFP instructions.  The second issue
requires avoiding any `_Decimal*' types, since they are not supported by
Clang.