runtime: fix crash on Plan 9

Since CL 46037, the runtime is crashing after calling
exitThread on Plan 9.

The exitThread function shouldn't be called on
Plan 9, because the system manages thread stacks.

Fixes #22221.

Change-Id: I5d61c9660a87dc27e4cfcb3ca3ddcb4b752f2397
Reviewed-on: https://go-review.googlesource.com/70190
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/proc.go

@@ -1170,10 +1170,10 @@ func mstart() {
 	mstart1(0)
 
 	// Exit this thread.
-	if GOOS == "windows" || GOOS == "solaris" {
-		// Windows and Solaris always system-allocate the
-		// stack, but put it in _g_.stack before mstart, so
-		// the logic above hasn't set osStack yet.
+	if GOOS == "windows" || GOOS == "solaris" || GOOS == "plan9" {
+		// Window, Solaris and Plan 9 always system-allocate
+		// the stack, but put it in _g_.stack before mstart,
+		// so the logic above hasn't set osStack yet.
 		osStack = true
 	}
 	mexit(osStack)