Fix handling of segment register faults when running with SVS. The behavior
is changed also in the non-SVS case.
I've put a documentation in amd64_trap.S. Basically, the problem with SVS
is that if iret faults, we already have a full trapframe pushed on the
stack and the CPU will push another frame on this stack (nested), but it
hits the redzone below the stack since it is still running with the user
page table loaded.
To fix that, we pop a good part of the trapframe earlier in intrfastexit.
If iret faults, the current %rsp has enough room for an iret frame, and
the CPU can push that without problem. We then switch back to the outer
iret frame (the frame the CPU was trying to pop by executing iret, but that
it didn't pop for real because iret faulted), call INTRENTRY, and handle
the trap as if it had been received from userland directly.
UnifiedSplitRaw
