v68k-callbacks:  Remove unused pop_args()

We no longer have any callbacks using the Mac Pascal calling convention.
v68k-mac:  Don't clobber A0 for Toolbox traps

Some Toolbox traps are register-based and expect an argument in A0, such
as _PtrToXHand, _HandAndHand, and _PtrAndHand.
v68k-mac:  Test D0 before return from OS trap

Inside Macintosh documents the trap dispatcher as doing this, so calling
code can follow a trap call immediately with a conditional branch.