Fix i386 atanhl (sNaN) (bug 20219).

The i386 version of atanhl returns sNaN for sNaN input.  This patch
fixes it to add NaN arguments to themselves so it returns qNaN in this
case.

Tested for x86_64 and x86.

	[BZ #20219]
	* sysdeps/i386/fpu/e_atanhl.S (__ieee754_atanhl): Add NaN argument
	to itself.
	* math/libm-test.inc (atanh_test_data): Add sNaN tests.