Closes #84689. Adding @lntue for review. I was curious about the implementation of `round_using_current_rounding_mode` used for the `nearbyint` functions. It has one of the rounding modes as unreachable ([here](https://github.com/llvm/llvm-project/blob/main/libc/src/__support/FPUtil/NearestIntegerOperations.h#L243)), and I was wondering if this was okay for the `nearbyint` functions. --------- Co-authored-by: Michael Flanders <mkf727@cs.washington.edu>