ptrace: add error stub to prevent mcexec from waiting

In LTP's ptrace03 test,
there is a exit() preceded by ptrace(PTRACE_TRACEME).
In this case, processes will be usually finished immediately.
But mcexec will fall into waiting for parent's ptrace(PTRACE_CONT),
because mcexec sends itself a signal in processing of exit().

arch/x86/kernel/include/syscall_list.h

@@ -55,6 +55,7 @@ SYSCALL_DELEGATED(79, getcwd)
 SYSCALL_DELEGATED(89, readlink)
 SYSCALL_DELEGATED(96, gettimeofday)
 SYSCALL_HANDLED(97, getrlimit)
+SYSCALL_HANDLED(101, ptrace)
 SYSCALL_DELEGATED(102, getuid)
 SYSCALL_DELEGATED(104, getgid)
 SYSCALL_DELEGATED(107, geteuid)