Z_ISR_DECLARE: update for C++ support
Z_ISR_DECLARE: update for C++ support

The implicit conversion of pointer-to-function to pointer-to-void is
not acceptable in C++.  Provide a C-style explicit cast to force the
reinterpretation.

Signed-off-by: Peter Bigot <a4112a160b558d5427af084318038e19a95979ee@nordicsemi.no>

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