Improve direct DPMI switch jmp for x86-64: don't use instructions that change
flags after popf, eliminate use of r10, and tell gcc that r8 and r9 are
clobbered.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@1616 7758b857-130e-0410-ab9c-d8735a528d18