Fix ldbl-128ibm expm1l (sNaN) (bug 20233).

The ldbl-128ibm version of expm1l returns sNaN for sNaN input.  This
patch fixes it to add such inputs to themselves so that qNaN is
returned in this case.

Tested for powerpc.

	[BZ #20233]
	* sysdeps/ieee754/ldbl-128ibm/s_expm1l.c (__expm1l): Add NaN input
	to itself.