[IA64] add missing syscall trace clear

The ptrace misses clearing the syscall trace flag.
The increased syscall overhead is retained after the trace is finished.
This case happens when strace is terminated by force.

Signed-off-by: Akiyama, Nobuyuki <akiyama.nobuyuk@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>