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.