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.

test/asm/use-label-outside-section.out

@@ -1 +1,2 @@
 $0
+$0