runtime: fix Plan 9 build

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/138050043

src/pkg/runtime/os_plan9.c

@@ -317,12 +317,6 @@ runtime·semawakeup(M *mp)
 	runtime·plan9_semrelease(&mp->waitsemacount, 1);
 }
 
-void
-os·sigpipe(void)
-{
-	runtime·throw("too many writes on closed pipe");
-}
-
 static int64
 atolwhex(byte *p)
 {

src/pkg/runtime/os_plan9.go

@@ -28,3 +28,7 @@ func errstr() string
 const stackSystem = 512
 
 type _Plink uintptr
+
+func os_sigpipe() {
+	gothrow("too many writes on closed pipe")
+}