Fix stack misalignment issues.

- Call ABI_AlignStack even on x86-64.

- Have ABI_AlignStack respect the difference in current alignment
  between the root JIT function, which has a prolog, and
  ProtectFunction thunks, which do not.  This was causing many games
  to crash on start on OS X.  Since this might otherwise mean changing
  the stack pointer before every call...

- Have one prolog/epilog function rather than two (one of which
  definitely did not do what it was thought to do), and make it
  actually work like a normal one, so that the stack frame shows up
  properly in the debugger.  There should be no performance impact.