xtensa: add calling entry point for multi-processing
xtensa: add calling entry point for multi-processing

Under multi-processing, only the first CPU#0 needs to go through
setting up the kernel structs and clearing out BSS (among others).
There is no need for other CPUs to do those tasks. Since each
Xtensa core starts using the same boot vector, CPUs other than #0
need to skip all the startup tasks by not calling to z_cstart().
So provide another entry point for those CPUs. Note that Xtensa
arch is highly configurable. So the implementation of the entry
point is up to each individual SoC config.

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr