As of version 0.9.8, Free Pascal supports the Delphi stdcall modifier. This modifier does actually nothing, since the Free Pascal compiler by default pushes parameters from right to left on the stack, which is what the modifier does under Delphi (which pushes parameters on the stack from left to right).
More information about this modifier can be found in the Programmer's guide in the section on the calling mechanism and the chapter on linking.