kernel: init: allow custom switch to main for no-multithreading case

Certain architectures, such as ARM Cortex-M require a custom
switch to main(), in case the case we build Zephyr without
support for multithreading (CONFIG_MULTITHREADING=n). So as
to keep the change to kernel/init.c as unintrusive as
possible, we install an ARCH-specific hook in z_cstart(),
which gets called for architectures that require this
customized switch to main() function.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>