x86: allow machine_crash_shutdown to be replaced

This patch a llows machine_crash_shutdown to
be replaced, just like any of the other functions
in machine_ops

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --cc arch/x86/kernel/crash.c
Simple merge

diff --cc arch/x86/kernel/reboot.c
Simple merge

diff --cc include/asm-x86/reboot.h
Simple merge