[libc] [llvm] [libc][math] Improve atanf performance. (PR #85463)

[via llvm-commits]

zimmermann6 wrote:

with the added exceptional case, all tests do pass with the CORE-MATH test suite. And the performance is almost the same as that of CORE-MATH. Good work!

https://github.com/llvm/llvm-project/pull/85463