os: give race detector chance to override Exit(0)

Racy tests do not fail currently, they do os.Exit(0).
So if you run go test without -v, you won't even notice.
This was probably introduced with testing.TestMain.

Racy programs do not have the right to finish successfully.

Change-Id: Id133d7424f03d90d438bc3478528683dd02b8846
Reviewed-on: https://go-review.googlesource.com/4371
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/proc.go

@@ -105,6 +105,14 @@ func main() {
 	}
 }
 
+// os_beforeExit is called from os.Exit(0).
+//go:linkname os_beforeExit os.runtime_beforeExit
+func os_beforeExit() {
+	if raceenabled {
+		racefini()
+	}
+}
+
 // start forcegc helper goroutine
 func init() {
 	go forcegchelper()