ldbl-128ibm-compat: Reuse tests for err.h and error.h functions

Commit IDs 9771e6cb and 7597b0c7 added tests for the functions
from err.h and error.h that can take long double parameters.
Afterwards, commit ID f0eaf862 reused them on architectures that
changed the long double format from the same as double to something else
(i.e.: architectures that imply ldbl-opt).  This patch reuses it again
for IEEE long double on powerpc64le.

Tested for powerpc64le.

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