[libc][math][c23] Add nextup{,f,f128} and nextdown{,f,f128} functions (#85431)

See https://github.com/llvm/llvm-project/issues/85283.

I had a test for `nextdownl` that was failing and I thought I should add
`nextupl` and `nextdownl` later and first make a PR for the other
functions.

cc @lntue