Fix dbl-64 atan in non-default rounding modes (bug 18197).

Fix dbl-64 atan in non-default rounding modes (bug 18197).

The dbl-64 implementation of atan does computations that expect to run
in round-to-nearest mode, and in other modes the errors can accumulate
to more than the maximum accepted 9ulp.  This patch makes it use
FE_TONEAREST internally, similar to other functions with such issues.

Tested for x86_64 and x86; no ulps updates needed.

	[BZ #18197]
	* sysdeps/ieee754/dbl-64/s_atan.c: Include <fenv.h>.
	(atan): Set FE_TONEAREST mode for internal computations.
	* math/auto-libm-test-in: Add more tests of atan.
	* math/auto-libm-test-out: Regenerated.