cmd/8c: disable use of prefetch with GO386=387

Fixes #4798.

R=ken2
CC=golang-dev
https://golang.org/cl/7323061

src/cmd/8c/txt.c

@@ -1396,6 +1396,9 @@ gprefetch(Node *n)
 {
 	Node n1;
 	
+	if(strcmp(getgo386(), "sse2") != 0) // assume no prefetch on old machines
+		return;
+
 	regalloc(&n1, n, Z);
 	gmove(n, &n1);
 	n1.op = OINDREG;