ldbl-128ibm-compat: Test positional arguments

The format string can request positional parameters, instead of relying
on the order in which they appear as arguments.  Since this has an
effect on how the type of each argument is determined, this patch
extends the test cases to use positional parameters with mixed double
and long double types, to verify that the IEEE long double
implementations of *printf work correctly in this scenario.

Tested for powerpc64le.

Reviewed-By: Paul E. Murphy <murphyp@linux.ibm.com>