Support symbols in .at address field

asm.js

@@ -550,15 +550,10 @@ class Parser {
 
     token_at_addr(token) {
         const value = this.integer(token);
-        if (value == null) {
-            console.error(
-                `ERROR: Unexpected token ${token} in .mem: `
-                    + "expected address");
-            this.at = undefined;
-            return;
-        }
-
-        this.at.addr = value;
+        if (value != null)
+            this.at.addr = value;
+        else
+            this.at.addr_symbol = token;
         const action = { type: Action.AT, at: this.at };
         this.at = undefined;
         this.state = State.TOP;
@@ -998,7 +993,8 @@ export class Assembler {
             return;
         }
         this.pos.mem = mem;
-        this.pos.addr = action.at.addr;
+        this.pos.addr = action.at.addr
+            ?? this.lookup_def(action.at.addr_symbol);
         this.data.push({ loc: { ...this.pos }, data: [] })
     }