kernel: make z_is_idle_thread_entry take code ptr

[RobinKastberg]

This is causing errors on IAR toolchain.

Talking to the C standard and compiler folks, they don't like this, since it is not guaranteed that code and data pointers are the same size. However if we do the cast it goes through fine.

       return entry_point == idle;
                             ^
"/workdir/zephyr/kernel/include/kthread.h",230  Warning[Pa131]: this is a function pointer constant. Did you intend a function call?

       return entry_point == idle;
                          ^
"/workdir/zephyr/kernel/include/kthread.h",230  Error[Pe042]: operand types are incompatible ("void *" and "void (*)(void *, void *, void *)")

[andyross]

Just declare the function to take k_thread_entry_t, which is a function pointer type we already have, and which all users of this validation framework should be using already.

[RobinKastberg]

changed the parameter type instead. This should be more correct