cmd/6c, cmd/6g, cmd/cc: fix undefined behavior warnings

Update #5764

Like Tribbles, the more you kill, the more spring up in their place.

R=rsc
CC=golang-dev
https://golang.org/cl/13324049

src/cmd/6c/sgen.c

@@ -36,7 +36,7 @@ gtext(Sym *s, int32 stkoff)
 {
 	vlong v;
 
-	v = (argsize() << 32) | (stkoff & 0xffffffff);
+	v = ((uvlong)argsize() << 32) | (stkoff & 0xffffffff);
 	if((textflag & NOSPLIT) && stkoff >= 128)
 		yyerror("stack frame too large for NOSPLIT function");
 

src/cmd/6g/ggen.c

@@ -596,7 +596,7 @@ dodiv(int op, Node *nl, Node *nr, Node *res)
 	check = 0;
 	if(issigned[t->etype]) {
 		check = 1;
-		if(isconst(nl, CTINT) && mpgetfix(nl->val.u.xval) != -1LL<<(t->width*8-1))
+		if(isconst(nl, CTINT) && mpgetfix(nl->val.u.xval) != -(1ULL<<(t->width*8-1)))
 			check = 0;
 		else if(isconst(nr, CTINT) && mpgetfix(nr->val.u.xval) != -1)
 			check = 0;

src/cmd/cc/scon.c

@@ -186,7 +186,7 @@ evconst(Node *n)
 		break;
 
 	case OASHL:
-		v = l->vconst << r->vconst;
+		v = (uvlong)l->vconst << r->vconst;
 		break;
 
 	case OLO: