xtensa: fix tickless idle build

_power_save_idle_exit() was removed long ago. All arches now just
call _sys_power_save_idle_exit() if PM is enabled.

Change-Id: I9cce3eecc8cbf1cbce15a355be420e747fb978de
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>