target-i386: rework CPL checks during task switch, preparing for next patch

During task switch, all of CS.DPL, CS.RPL, SS.DPL must match (in addition
to all the other requirements) and will be the new CPL.  So far this worked
by carefully setting the CS selector and flags before doing the task
switch; but this will not work once we get the CPL from SS.DPL.

Temporarily assume that the CPL comes from CS.RPL during task switch
to a protected-mode task, until the descriptor of SS is loaded.

Tested-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
