gopack: work around gcc bug in hash function

Fixes #48.  (this time for sure!)

R=r, r1
https://golang.org/cl/152088

src/cmd/gopack/ar.c

@@ -784,9 +784,16 @@ hashstr(char *name)
 	h = 0;
 	for(cp = name; *cp; h += *cp++)
 		h *= 1119;
-	if(h < 0)
+	
-		h = ~h;
+	// the code used to say
-	return h;
+	//	if(h < 0)
+	//		h = ~h;
+	// but on gcc 4.3 with -O2 on some systems,
+	// the if(h < 0) gets compiled away as not possible.
+	// use a mask instead, leaving plenty of bits but
+	// definitely not the sign bit.
+
+	return h & 0xfffffff;
 }
 
 int