[XEN] Implement faster int 0x80 handling for compat mode guests.

[XEN] Implement faster int 0x80 handling for compat mode guests.

Using the GPF handler to spot the software interrupt and pass it back
to the guest increases the base syscall time by a factor of 2.7
compared with 32on32 using direct trap to ring 1. (0.3270->0.8680
microseconds, measured with lmbench lat_syscall).

Since the 64 bit IDT can only contain 64 bit segment selectors we
cannot trap directly to compat mode ring 1. However implementing a
dedicated 64 bit ring 0 trap handler allows us to avoid much of the
GPF handler overhead and reduces the overhead to 1.7 times
(0.3270->0.5497 microseconds).

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>