Implement nearbyint/floor/ceil backup functions

[ErikMcClure]

On most modern x86-64 CPUs, floor/ceil/nearbyint intrinsics all get turned into CPU instructions. However, on older CPUs or low-power architectures, LLVM eventually gives up and simply calls the C standard library functions floorf()/ceilf()/nearbyintf(). Of course, inNative doesn't have a C standard library, so this fails.

A version of these functions should be provided in the default environment so the fallback actually works.