[libc][math][c23] Improve FPU path for C23 math function rsqrtf16

In #160639 it was discovered that [FPU path had lower throughput](https://github.com/llvm/llvm-project/pull/160639#issuecomment-4637223439) than approximation path.

I would like to see how using approach described in https://github.com/llvm/llvm-project/blob/main/libc/src/__support/FPUtil/sqrt.h we can potential improve the throughput of that function, and later re-use that + approximation idea for the follow-up `rsqrt` PRs for higher-precision data types.

Please assign me to this issue, thanks!