syscall: make Exit call runtime.exit

syscall.Exit and runtime.exit do the same thing.
Why duplicate code?

CL 45115 fixed bug where windows runtime.exit was correct,
but syscall.Exit was broken. So CL 45115 fixed windows
syscall.Exit by calling runtime.exit.

Austin suggested that all OSes should do the same, and
this CL implements his idea.

While making changes, I discovered that nacl syscall.Exit
returned error

func Exit(code int) (err error)

and I changed it into

func Exit(code int)

like all other OSes. I assumed it was a mistake and it
is OK to do because cmd/api does not complain about it.

Also I changed plan9 runtime.exit to accept int32 just
like all other OSes do.

Change-Id: I12f6022ad81406566cf9befcc6edc382eebd413b
Reviewed-on: https://go-review.googlesource.com/66170
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/runtime/os_plan9.go

@@ -393,7 +393,7 @@ func postnote(pid uint64, msg []byte) int {
 }
 
 //go:nosplit
-func exit(e int) {
+func exit(e int32) {
 	var status []byte
 	if e == 0 {
 		status = emptystatus