Fix dbl-64 atan2 (sNaN, qNaN) (bug 20252).

Fix dbl-64 atan2 (sNaN, qNaN) (bug 20252).

The dbl-64 implementation of atan2, passed arguments (sNaN, qNaN),
fails to raise the "invalid" exception.  This patch fixes it to add
both arguments, rather than just adding the second argument to itself,
in the case where the second argument is a NaN (which is checked for
before checking for the first argument being a NaN).  sNaN tests for
atan2 are added, along with some qNaN tests I noticed were missing but
should have been there by analogy with other tests present.

Tested for x86_64 and x86.

	[BZ #20252]
	* sysdeps/ieee754/dbl-64/e_atan2.c (__ieee754_atan2): Add both
	arguments when second argument is a NaN.
	* math/libm-test.inc (atan2_test_data): Add sNaN tests and more
	qNaN tests.