5l: fix arm linker bug introduced by 4742041

Should fix the arm build.

R=golang-dev, dsymonds, adg
CC=golang-dev
https://golang.org/cl/4777041

src/cmd/5l/span.c

@@ -231,7 +231,7 @@ span(void)
 				*bp++ = v;
 				*bp++ = v>>8;
 				*bp++ = v>>16;
-				*bp = v>>24;
+				*bp++ = v>>24;
 			}
 		}
 	}