The 386 callback trampoline returned only EAX, losing the upper half
of int64 and uint64 results. Its callbackArgs area also overlapped
the copied C arguments.

Reserve space for the full callbackArgs struct, move the argument
copy past it, and return both result words in EDX:EAX.

Add regression tests for signed and unsigned 64-bit callback results,
including a callback with stack arguments.

Closes #524