[IA64] enable singlestep on system call

As is pointed out in
http://www.gelato.org/community/view_linear.php?id=1_1036&from=authors&value=Ian%20Wienand#1_1039

,
if single step on break instruction, the break fault has higher
priority than the single-step trap. When the break fault handler
is entered, it advances the IP by 1 instruction so break instruction
single-stepping is skipped, actually it is next instruction which
is single stepped.

This patch modifies this, it adds TIF_SINGLESTEP bit for thread
flags, and generate a fake sigtrap when single stepping break
instruction. Test case in attachment can verify this. Any comments
is welcome.

Signed-off-by: bibo, mao <bibo.mao@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>