[libc] [llvm] [libc][math][c23] Add {,u}fromfp{,x}{,f,l,f128} functions (PR #86003)

[via llvm-commits]

overmighty wrote:

You're welcome.

By the way, `roundeven` could be implemented similarly to `fputil::round` and the branches for `FP_INT_TONEAREST` in `fputil::round_using_specific_rounding_mode`. I can work on this later if you want.

https://github.com/llvm/llvm-project/pull/86003