Ensure native-compilation support can be found in Emacs 30 and refactor slightly

The `foundp` check on each of the native-compilation functions failed on Emacs 30 without requiring *comp.el*. Further it's better to use the builtin `native-comp-available-p` function instead of using foundp as it doesn't depend on potentially version specific semantic as a function might not be found but native-compilation would be still available. Fixes `Emacs 30.0.50 does not support native compilation`.