Fix i386/x86_64 log2l (sNaN) (bug 20235).

The i386/x86_64 versions of log2l return sNaN for sNaN input.  This
patch fixes them to add NaN inputs to themselves so that qNaN is
returned in this case.

Tested for x86_64 and x86.

	[BZ #20235]
	* sysdeps/i386/fpu/e_log2l.S (__ieee754_log2l): Add NaN input to
	itself.
	* sysdeps/x86_64/fpu/e_log2l.S (__ieee754_log2l): Likewise.
	* math/libm-test.inc (log2_test_data): Add sNaN tests.