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Allow address expressions in ?c after int casts.

This fixes ocaml on non-x86 architectures, where we have code
that looks like:

	#define Fl_head ((uintptr_t)(&sentinel.first_field))

Without this change, we get an error about a non-constant
initializer. This change takes the checks for pointers and
makes them apply to all expressions. It also makes the checks
stricter, preventing the following from compiling to junk:

	int x;
	int y = 42;
	int *p = &x + y

--- a/sys/src/cmd/cc/dcl.c

+++ b/sys/src/cmd/cc/dcl.c

@@ -373,35 +373,35 @@

 			goto gext;

 		}

 		if(t->etype == TIND) {

-			while(a->op == OCAST) {

+			if(a->op == OCAST)

 				warn(a, "CAST in initialization ignored");

-				a = a->left;

-			}

-			if(!sametype(t, a->type)) {

+			if(!sametype(t, a->type))

 				diag(a, "initialization of incompatible pointers: %s\n%T and %T",

 					s->name, t, a->type);

-			}

-			switch(a->op) {

-			case OADDR:

-				a = a->left;

-				break;

-			case ONAME:

-			case OIND:

-				diag(a, "initializer is not a constant: %s", s->name);

-				return Z;

-			}

-			goto gext;

 		}

 		while(a->op == OCAST)

 			a = a->left;

-		if(a->op == OADDR) {

-			warn(a, "initialize pointer to an integer: %s", s->name);

+

+		switch(a->op) {

+		case OADDR:

+			if(t->etype != TIND)

+				warn(a, "initialize pointer to an integer: %s", s->name);

 			a = a->left;

-			goto gext;

+			break;

+		case OADD:

+			/*

+			 * Constants will be folded before this point, which just leaves offsets

+			 * from names.

+			 */

+			l = a->left;

+			r = a->right;

+			if(l->op == OADDR && r->op == OCONST || r->op == OADDR && l->op == OCONST)

+				break;

+		default:

+			diag(a, "initializer is not a constant: %s", s->name);

+			return Z;

 		}

-		diag(a, "initializer is not a constant: %s", s->name);

-		return Z;

 	gext:

 		gextern(s, a, o, t->width);