gas/expr.c: 0b vs 0b0 vs 00b

[DJ Delorie]

> Actually 1b is a backward reference too.  I think both 0b and 1b
> without following binary digits ought to be treated as backward refs
> on these targets, since if you want to write a binary 0 or 1 with an
> explicit suffix that can be done as 0B or 1B.

Note that the gas docs are vague about this: one spot says local refs
are '1' through '9', another refers to the "first ten single-digit..."

But yeah, I was only trying to special-case the 0b[01]+ case.

> Here's a patch that extends yours a little.  What do you think?

Seems OK to me.