arc: fixes a missing declaration compile error

Building microkernel for ARC causes a compiler error
due to missing declaration of _sys_idle_elapsed_ticks.
Declare this as extern.

Jira: ZEP-397
Change-Id: I83701f693fea0fcb1ae2c30e0c52abe7987de1f1
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>