xenserver: Turn on SSE and SSE2 for the build, for atomic 64-bit ops.

The ovs-atomic-i586 implementation of atomic operations can implement
64-bit atomics more efficiently when SSE is supported.  XenServer runs only
on 64-bit capable processors, in 32-bit mode, so we know on XenServer that
SSE and SSE2 are supported because they are architectural for amd64.  Thus,
this commit enables SSE and SSE2 when building for XenServer to get the
improved atomics support.

I tested that this successfully adds -msse -msse2 to the compiler flags
inside a XenServer DDK, but I didn't actually run it on a real XenServer
install.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>