[libc-commits] [libc] [libc][math] implement `signbit` (PR #97791)

[via libc-commits]

akielaries wrote:

As mentioned in the original thread for #96322 [here](https://github.com/llvm/llvm-project/issues/96322#issuecomment-2183411373), I can add the macros + tests for these operations as well either in this PR or an additional one

https://github.com/llvm/llvm-project/pull/97791