ANDROID: GKI: panic: add vendor callback function in panic()

Each vendor might want to implement some debug code when the kernel
panics. So, add a vendor_panic_cb callback for vendors to implement.

Bug: 149258398
Test: compile
Change-Id: I7a374b0089f72c2511db6fe3b8cdd18f41a1eb6c
Signed-off-by: Saravana Kannan <saravanak@google.com>
(cherry picked from commit 911d9c70c2c50b0383ed0b652bb84ca8832e4a2b)
Signed-off-by: Will McVicker <willmcvicker@google.com>
[willmcvicker: only pulled in the ABI diffs]