Use IRET to jump into DPMI client code on x86-64. It's nice because it pops
up ss:esp even for same-privilege transfers from long mode. To avoid the
infamous 16-bit stack ESP corruption issue the iret stack frame is allocated
at 0x100000000 or a similar address with bits 16-31 zero.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@1620 7758b857-130e-0410-ab9c-d8735a528d18