[libc][math] Adds entrypoint and tests for nearbyintf128,scalbnf128 (#88443)

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.

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Co-authored-by: Michael Flanders <mkf727@cs.washington.edu>