s390x regtest: Rework dfp-2

s390x regtest: Rework dfp-2

The dfp-2 test case has two issues:

* It doesn't test the rounding modes correctly.  Since the inline assembly
  uses a "d" constraint to pass the rounding mode, a random register
  number is placed into the instruction's rounding mode field instead of
  the intended rounding mode.

* It cannot be compiled with Clang.  (See Bug 465782.)

Fix these with a rewrite.  Fixing the first issue also requires adjusting
the test case output `dfp-2.stdout.exp', while fixing the second requires
avoiding `_Decimal*' data types.  Also reduce some code duplication.