[runtime.c] support platform specific implementation of main function

By default, the main function is calling a generated behavior. Alternatively, the user can skip the generated code via undef ET_RUNNER_ACTIVATE. To customize the main behavior, you then implement etUserPreRun(void).

This is not the most intuitive approach. Better would be to provide an explicit switch and add a life cycle method for the main function, which has the main arg pointer. This improves the flexibility and readability of platform implementation.

To avoid breaking changes, generated code should be backwards compatible.