[IA64] find thread for user rbs address

I encountered one problem when running ptrace test case the situation
is this: traced process's syscall parameter needs to be accessed, but
for sys_clone system call with clone_flag (CLONE_VFORK | CLONE_VM |
SIGCHLD) parameter.  This syscall's parameter accessing result is wrong.

The reason is that vforked child process mm point is the same, but
tgid is different. Without this patch find_thread_for_addr will return
vforked process if vforked process is also stopped, but not the thread
which calls vfork syscall.

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