s390x regtest: Reduce pfpo test case output

s390x regtest: Reduce pfpo test case output

The `pfpo' test case prints floating-point values with the "%f" and "%Lf"
formats.  Sometimes also "%lf" is used, but that's equivalent to "%f"
since the "l" length modifier doesn't affect floating-point formatting.

These "f"-style formats are not well suited for such a test case:

* They generate excessively long output for numbers with large absolute
  values.  This makes `pfpo.stdout.exp' difficult to handle.

* They can lose precision, in particular for numbers close to zero, where
  they just print zeros.

Use the formats "%a" and "%La" instead and adjust pfpo.stdout.exp
accordingly.