[IA64] kexec: Move machine_shutdown from machine_kexec.c to process.c

This moves the ia64 implementation of machine_shutdown() from
machine_kexec.c to process.c, which is in keeping with the implelmentation
on other architectures, and seems like a much more appropriate home for it.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>