kexec: migrate to reboot cpu

Commit 1b3a5d02 ("reboot: move arch/x86 reboot= handling to generic
kernel") moved reboot= handling to generic code.  In the process it also
removed the code in native_machine_shutdown() which are moving reboot
process to reboot_cpu/cpu0.

I guess that thought must have been that all reboot paths are calling
migrate_to_reboot_cpu(), so we don't need this special handling.  But
kexec reboot path (kernel_kexec()) is not calling
migrate_to_reboot_cpu() so above change broke kexec.  Now reboot can
happen on non-boot cpu and when INIT is sent in second kerneo to bring
up BP, it brings down the machine.

So start calling migrate_to_reboot_cpu() in kexec reboot path to avoid
this problem.

Bisected by WANG Chao.

Reported-by: Matthew Whitehead <mwhitehe@redhat.com>
Reported-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Tested-by: Baoquan He <bhe@redhat.com>
Tested-by: WANG Chao <chaowang@redhat.com>
Acked-by: H. Peter Anvin <hpa@linux.intel.com...>