With help from Joseph Myers.
* sysdeps/ieee754/ldbl-128/s_atanl.c (__atanl): Handle tiny and
very large arguments properly.
* math/libm-test.inc (atan_test): New tests.
(atan2_test): New tests.
* sysdeps/sparc/fpu/libm-test-ulps: Update.
* sysdeps/x86_64/fpu/libm-test-ulps: Update.