runtime: only use a single variable in USED

The gccgo version of USED only accepts a single variable, so
this simplifies merging.

LGTM=minux, dave
R=golang-codereviews, minux, dave
CC=golang-codereviews
https://golang.org/cl/115630043

src/pkg/runtime/netpoll_epoll.c

@@ -55,7 +55,8 @@ runtime·netpollclose(uintptr fd)
 void
 runtime·netpollarm(PollDesc* pd, int32 mode)
 {
-	USED(pd, mode);
+	USED(pd);
+	USED(mode);
 	runtime·throw("unused");
 }