Do not allow defining labels outside sections (#2013)

Creating a `Symbol` with `type = SYM_LABEL` but `section = nullptr`
is inconsistent and dangerous. I was not able to cause any buggy
behavior so far, but it's safer and reasonable to not create such
a symbol in the first place.

The main consequence is that `DEF(LabelOutsideSection)` will now
evaluate as 0.

src/asm/rpn.cpp

@@ -227,9 +227,9 @@ static int32_t tryConstMask(Expression const &lhs, Expression const &rhs) {
 		return -1;
 	}
 
-	assume(sym.isNumeric());
+	assume(sym.type == SYM_LABEL);
+	// We can now safely use `expr.value()` and `sym.getSection()`
 
-	// We can now safely use `expr.value()`
 	int32_t mask = expr.value();
 
 	// The mask must not cover any unknown bits