[IA64] pvops: paravirtualize entry.S

paravirtualize ia64_swtich_to, ia64_leave_syscall and ia64_leave_kernel.
They include sensitive or performance critical privileged instructions
so that they need paravirtualization.
To paravirtualize them by single source and multi compile
they are converted into indirect jump. And define each pv instances.

Cc: Keith Owens <kaos@ocs.com.au>
Cc: "Dong, Eddie" <eddie.dong@intel.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>