Add accessor for SIGTRAP cause in wait status

R=rsc
APPROVED=rsc
DELTA=7  (7 added, 0 deleted, 0 changed)
OCL=31563
CL=31565

src/pkg/syscall/syscall_linux.go

@@ -164,6 +164,13 @@ func (w WaitStatus) StopSignal() int {
 	return int(w >> shift) & 0xFF;
 }
 
+func (w WaitStatus) TrapCause() int {
+	if w.StopSignal() != SIGTRAP {
+		return -1;
+	}
+	return int(w >> shift) >> 8;
+}
+
 //sys	wait4(pid int, wstatus *_C_int, options int, rusage *Rusage) (wpid int, errno int)
 func Wait4(pid int, wstatus *WaitStatus, options int, rusage *Rusage) (wpid int, errno int) {
 	var status _C_int;