ldbl-128ibm-compat: Add err.h functions

Use the recently added, internal functions, __vwarnx_internal and
__vwarn_internal, to provide err.h functions that can take long double
arguments with IEEE binary128 format on platforms where long double can
also take double format or some non-IEEE format (currently, this means
powerpc64le).

Tested for powerpc64le.

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