s390x regtest: Rework dfpconv

s390x regtest: Rework dfpconv

The dfpconv test case has two major issues:

* It truncates certain 64-bit integers to 32 bits when using them as
  inputs or outputs to inline assemblies.  In particular this leads to
  wrong output.

* It uses `_Decimal*' types and thus cannot be compiled with Clang.

Fix both.  Also, remove a duplicate from the set of tested numbers, remove
the dependency on "opcodes.h", and fix indentation.  Since the test output
file dfpconv.stdout.exp is affected by these changes, adjust it
accordingly.