Fix dbl-64 acos (sNaN) (bug 20212).

The dbl-64 version of acos returns sNaN for sNaN arguments.  This
patch fixes it to add NaN arguments to themselves so that qNaN is
returned in this case.

Tested for x86_64 and x86.

	[BZ #20212]
	* sysdeps/ieee754/dbl-64/e_asin.c (__ieee754_acos): Add NaN
	argument to itself.
	* math/libm-test.inc (acos_test_data): Add sNaN tests.