ANDROID: GKI: Add stub __cpu_isolated_mask symbol

Some vendor modules might want to keep track of isolated CPUs. Add a
stub symbol that never isolates any CPU.

Bug: 149816871
Signed-off-by: Saravana Kannan <saravanak@google.com>
Change-Id: Ia494314168e94d72b0c1e8b786c150b9403dbf1a