sys_clock: Fix unsafe tick count usage

The system tick count is a 64 bit quantity that gets updated from
interrupt context, meaning that it's dangerously non-atomic and has to
be locked.  The core kernel clock code did this right.

But the value was also exposed to the rest of the universe as a global
variable, and virtually nothing else was doing this correctly.  Even
in the timer ISRs themselves, the interrupts may be themselves
preempted (most of our architectures support nested interrupts) by
code that wants to set timeouts and inspect system uptime.

Define a z_tick_{get,set}() API, eliminate the old variable, and make
sure everyone uses the right mechanism.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

libmetal/lib/system/zephyr/time.c

@@ -12,10 +12,8 @@
 #include <metal/time.h>
 #include <sys_clock.h>
 
-extern volatile u64_t _sys_clock_tick_count;
-
 unsigned long long metal_get_timestamp(void)
 {
-	return (unsigned long long)_sys_clock_tick_count;
+	return (unsigned long long)z_tick_get();
 }