syscall: cleanly exit all Go procs on Plan 9

syscall.Exit would originally kill only the calling
Go proc, leaving behind other procs in the
same group. This change makes syscall.Exit call
runtime·exit, which due to CL
https://golang.org/cl/5617048
will cleanly exit all the Go procs in the group.

R=golang-dev, rsc, rminnich, remyoudompheng, ality, john
CC=golang-dev, mirtchovski
https://golang.org/cl/6036051

src/pkg/syscall/asm_plan9_386.s

@@ -149,3 +149,11 @@ copyresult6:
 	MOVSL
 	MOVSL
 	RET
+
+//func exit(code int)
+// Import runtime·exit for cleanly exiting.
+TEXT ·exit(SB),7,$4
+	MOVL	code+0(FP), AX
+	MOVL	AX, 0(SP)
+	CALL	runtime·exit(SB)
+	RET

src/pkg/syscall/syscall_plan9.go

@@ -69,23 +69,10 @@ func errstr() string {
 
 func Getpagesize() int { return 4096 }
 
-//sys	exits(msg *byte)
+// Implemented in assembly to import from runtime.
-func Exits(msg *string) {
+func exit(int)
-	if msg == nil {
-		exits(nil)
-	}
 
-	exits(StringBytePtr(*msg))
+func Exit(code int) { exit(code) }
-}
-
-func Exit(code int) {
-	if code == 0 {
-		Exits(nil)
-	}
-
-	msg := itoa(code)
-	Exits(&msg)
-}
 
 func readnum(path string) (uint, error) {
 	var b [12]byte