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

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lntue wrote:

About the `__builtin_unreachable`, currently we only support 4 rounding modes, leaving the `default` case as `__builtin_unreachable` will help us to detect if the values returned by `quick_get_round` is unexpected by using UBSAN build and test config.

https://github.com/llvm/llvm-project/pull/88443