powerpc/modules: Create module_trampoline_target()

ftrace has way too much knowledge of our kernel module trampoline
layout hidden inside it. Create module_trampoline_target() that gives
the target address of a kernel module trampoline.

Signed-off-by: Anton Blanchard <anton@samba.org>

diff --cc arch/powerpc/include/asm/module.h
Simple merge

diff --cc arch/powerpc/kernel/module_64.c
Simple merge