kernel: idle: fix build error on arm-clang

When building sample.minimal.mt-no-preempt-no-timers.arm on arm-clang
we get a link error as z_pm_save_idle_exit expects sys_clock_idle_exit
to be defined.

However the sample sets CONFIG_SYS_CLOCK_EXISTS=n so
sys_clock_idle_exit() will not be defined by any driver.  So add proper
ifdef protection in z_pm_save_idle_exit to fix this.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>