gas/macro: drop ISSEP()

I've been irritated by it more than once: It very obviously doesn't cover
all separators, hence resulting in inconsistent behavior. Nor do both use
sites look to really want the same set of separators. In macro_expand() we
really can pull the get_token() call ahead. If we don't find the expected
'=' we can simply continue parsing from the original position.

The use in get_any_string() is dead code afaict, inherited from gasp. The
sole leftover thereof is handled in the scrubber (see H_TICK_HEX,
LEX_IS_H, and alike there). With that dropped, ISBASE() also can be.