[libc][math][c23] Implement C23 math function rsqrtf16

[amemov]

Part of #95250. I checked the code, and past issues/PRs, and it seems no one is working on this.

I would like to work on this implementation, please assign me to it. Thank you!

[amemov]

@overmighty

[llvmbot]

@llvm/issue-subscribers-libc

Author: Anton (amemov)

Part of #95250. I checked the code, and past issues/PRs, and it seems no one is working on this.

I would like to work on this implementation, please assign me to it. Thank you!