[llvm] [libc] [libc][math] Add initial support for C23 float128 math functions, starting with copysignf128. (PR #71731)

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lntue wrote:

> overall LGTM, should we move the float16 stuff to its own PR?

I removed float16 stuff to be added in a separate PR.

https://github.com/llvm/llvm-project/pull/71731