On x86_64, we can defer %ds and %es reload to the heavyweight context switch,
since nothing in the lightweight paths uses the host %ds or %es (they are
ignored by the processor).  Furthermore we can avoid the load if the segments
are null, by letting the hardware load the null segments for us.  This is the
expected case.

On i386, we could avoid the reload entirely, since the entry.S paths take care
of reload, except for the SYSEXIT path which leaves %ds and %es set to __USER_DS.
So we set them to the same values as well.

Saves about 70 cycles out of 1600 (around 4%; noisy measurements).
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>