[IA64] speed up syscall path a bit more

Recently I noticed that clearing ar.ssd/ar.csd right before srlz.d is
causing significant stalling in the syscall path.  The patch below
fixes that by moving the register-writes after srlz.d.  On a Madison,
this drops break-based getpid() from 241 to 226 cycles (-15 cycles).

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