[S390] ptrace inferior call interactions with TIF_SYSCALL

The TIF_SYSCALL bit needs to be cleared if the debugger changes the state
of the ptraced process in regard to the presence of a system call.
Otherwise the system call will be restarted although the debugger set up
an inferior call.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>