fix ordering issue with recording clone(2)

The x86 clone(2) syscall has the prototype:

sys_clone(unsigned long flags, unsigned long newsp,
          int* parent_tid, int tls_val, int* child_tid);

Due to a quirk in the recorder, we were inverting the recording of the
tls_val (struct user_desc* for x86) and child_tid.

(cf. recorder.cc:handle_ptrace_event)

Fixes #1339.