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

Path from Rainer Orth.
    
    Reviewed-on: https://go-review.googlesource.com/c/153118

From-SVN: r266890

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-edc7e7172e674b8c7e9c3caa30e24280cd289a9c
+f0266d382f8965b7bcaa380aa963498c1884505e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/stubs_nonlinux.go

@@ -17,4 +17,5 @@ func gettid() _pid_t {
 
 func tgkill(pid _pid_t, tid _pid_t, sig uint32) uint32 {
 	throw("tgkill not implemented")
+	return 0
 }