runtime: add missing return for non-GNU/Linux version of tgkill

Path from Rainer Orth.

Change-Id: I40fc68a34a77a62c18eb5aa41a951c3885f55d37
Reviewed-on: https://go-review.googlesource.com/c/153118
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/libgo/go/runtime/stubs_nonlinux.go b/libgo/go/runtime/stubs_nonlinux.go
index 4cdab0c..325d35d 100644
--- a/libgo/go/runtime/stubs_nonlinux.go
+++ b/libgo/go/runtime/stubs_nonlinux.go
@@ -17,4 +17,5 @@
 
 func tgkill(pid _pid_t, tid _pid_t, sig uint32) uint32 {
 	throw("tgkill not implemented")
+	return 0
 }