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

[via llvm-commits]

zimmermann6 wrote:

I get an error with the CORE-MATH exhaustive tests:
```
Running exhaustive check in --rndu mode...
FAIL x=-0x1.fc5d82p+0 ref=-0x1.1ab2fp+0 y=-0x1.1ab2eep+0
```
Can you confirm?

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