[IA64] Replace stale KDB-code with useful MAGIC_SYSRQ code in simserial.c

Patch makes it possible to use the "F4" function key to do
magic sysrq in the HP Ski simulator.

Signed-off-by: David Mosberger-Tang <davidm@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>