x86: allow unary operators to start a memory operand

So far only - was permitted, but +, !, and ~ ought to be treated the
same.

Rather than adding them to digit_chars[], which was at least odd to have
held - so far, drop this array and its wrapper macro for being used just
once.

While adjusting this logic, also include [ in the characters which may
start a displacement expression - gas generally treats [] as equivalent
to ().