ldbl-128ibm-compat: Test double values

A single format string can take double and long double parameters at the
same time.  Internally, these parameters are routed to the same
function, which correctly reads them and calls the underlying functions
responsible for the actual conversion to string.  This patch adds a new
case to test this scenario.

Tested for powerpc64le.

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