entry routine load the real ESP0 off that per-cpu stack. Make this
even faster by putting the sysenter stack in the per-CPU TSS, so
that we can use the tss->esp0 value directly (which we have to
update on task switches anyway).

CAREFUL! This needs very subtle code for debug and NMI exceptions,
to make sure we don't run with the sysenter stack in any real kernel
code!