This is the last non-trivial user of btfixup.

Like sparc64, use a special patch section to resolve the various
implementations of how to read the current CPU's ID when we don't
have current_thread_info()->cpu necessarily available.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Tested-by: NSam Ravnborg <sam@ravnborg.org>