gas: sanitize FB- and dollar-label uses

gas: sanitize FB- and dollar-label uses

I don't view it as sensible to be more lax when it comes to references
to (uses of) such labels compared to their definition: The latter has
been limited to decimal numerics, while the former permitted any radix.
Beyond that leading zeroes on such labels aren't helpful either. Imo
labels and their use sites would better match literally, to avoid
confusion.

As it turns out, one z80 testcase actually had such an odd use of labels
where definition and use don't match in spelling. That testcase is being
adjusted accordingly.

While there also adjust a comment on a local variable in
integer_constant().