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

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overmighty wrote:

I added tests for the fallback to `FP_INT_TONEAREST` when the value of the `rnd` argument doesn't match the value of any math rounding direction macro.

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