Add tests for argp_error and argp_failure with floating-point parameters

The functions argp_error and argp_failure, from argp.h, have a format
string as parameter, which can possibly request the printing of
floating-point values.  These values could be of long double type, which
can have different formats, depending on the architecture and on
compilation parameters (for instance, on powerpc, long double values can
have double format (-mlong-double-64) or IBM Extended Precision format
(-mlong-double-128)).

This patch adds tests for argp_error and argp_failure that contain a
format string with double and long double conversion specifiers ('%f'
and '%Lf').  These tests automatically check that the default format of
the long double type works.  A future patch will extend the test for
platforms that can have an optional format for long double.

Tested for powerpc64le.

	* argp/Makefile (tests): Add tst-ldbl-argp.
	* argp/tst-ldbl-argp.c: New file.