powerpc/ftrace: Fix assembly trampoline register usage

commit fd5a42980e1cf327b7240adf5e7b51ea41c23437 upstream.

Just like the module loader, ftrace needs to be updated to use r12
instead of r11 with newer gcc's.

Signed-off-by: Roger Blofeld <blofeldus@yahoo.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>