ldbl-opt: Add argp_error and argp_failure (bug 23983)

The functions argp_error and argp_failure are missing support for
printing long double values when long double has the same format as
double.  This patch adds the new functions __nldbl_argp_error and
__nldbl_argp_failure, as well as header magic to redirect calls to them
when -mlong-double-64 is in use.

Tested for powerpc, powerpc64 and powerpc64le.