kexec: identify which cpu the kexec image is being executed on

kexec: identify which cpu the kexec image is being executed on

A patch to this effect has been in XenServer for a little while, and has
proved to be a useful debugging point for servers which have different
behaviours depending when crashing on the non-bootstrap processor.

Moving the printk() from kexec_panic() to one_cpu_only() means that it will
only be printed for the cpu which wins the race along the kexec path.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>