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.