gas/symbols: introduce md_resolve_symbol

Assuming GMSD is a special operand, marked as O_md1, the code:

    .set VREG, GMSD
    .set REG, VREG
    extsw REG, 2

...fails upon attempts to resolve the value of the symbol. This happens
since machine-dependent values are not handled in the giant op switch.
We introduce a custom md_resolve_symbol macro; the ports can use this
macro to customize the behavior when resolve_symbol_value hits O_md
operand.