arm64: break while loop if task had been rescheduled

While printing a task's backtrace and this task isn't
current task, it is possible that task's fp and fp+8
have the same value, so cannot break the while loop.
This can break while loop if this task had been
rescheduled during print this task's backtrace.

Change-Id: Ibcab9e570946b22869297695216d88368173b62a
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>