runtime: fix chan alignment on 32 bits

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews, khr, rsc
https://golang.org/cl/128700043

src/pkg/runtime/chan.h

@@ -21,6 +21,9 @@ struct	Hchan
 	byte*	buf;
 	uint16	elemsize;
 	uint32	closed;
+#ifndef GOARCH_amd64
+	uint32	pad;			// ensures proper alignment of the buffer that follows Hchan in memory
+#endif
 	Type*	elemtype;		// element type
 	uintgo	sendx;			// send index
 	uintgo	recvx;			// receive index