Fix i386/x86_64 scalbl with sNaN input (bug 20296).

The x86_64 and i386 versions of scalbl return sNaN for some cases of
sNaN input and are missing "invalid" exceptions for other cases.  This
results from overly complicated code that either returns a NaN input,
or discards both inputs when one is NaN and loads a NaN from memory.
This patch fixes this by simplifying the code to add the arguments
when either one is NaN.

Tested for x86_64 and x86.

	[BZ #20296]
	* sysdeps/i386/fpu/e_scalbl.S (__ieee754_scalbl): Add arguments
	when either argument is a NaN.
	* sysdeps/x86_64/fpu/e_scalbl.S (__ieee754_scalbl): Likewise.
	* math/libm-test.inc (scalb_test_data): Add sNaN tests.