Fix ldexp, scalbn, scalbln for sNaN input (bug 20225).

Fix ldexp, scalbn, scalbln for sNaN input (bug 20225).

The wrapper implementations of ldexp / scalbn / scalbln
(architecture-independent), and their float / long double variants,
return sNaN for sNaN input.  This patch fixes them to add relevant
arguments to themselves so that qNaN is returned in this case.

Tested for x86_64 and x86.

	[BZ #20225]
	* math/s_ldexp.c (__ldexp): Add non-finite or zero argument to
	itself.
	* math/s_ldexpf.c (__ldexpf): Likewise.
	* math/s_ldexpl.c (__ldexpl): Likewise.
	* math/w_scalbln.c (__w_scalbln): Likewise.
	* math/w_scalblnf.c (__w_scalblnf): Likewise.
	* math/w_scalblnl.c (__w_scalblnl): Likewise.
	* math/libm-test.inc (scalbn_test_data): Add sNaN tests.
	(scalbln_test_data): Likewise.