soc/intel_adsp_cavs15: Unbreak newlib linkage
soc/intel_adsp_cavs15: Unbreak newlib linkage

In commit efa05d1e42da ("soc/intel_adsp: Put initial stack into the
CPU0 interrupt stack") the "_end" symbol was accidentally removed from
the linker script.  Newlib needs this to size its heap.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

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