Fix ldbl-128ibm log1pl (sNaN) (bug 20234).

The ldbl-128ibm version of log1pl 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 #20234]
	* sysdeps/ieee754/ldbl-128ibm/s_log1pl.c (__log1pl): Add positive
	infinity or NaN input to itself.