lib/ovs-atomic-i586: Faster 64-bit atomics on 32-bit builds with SSE.

Aligned 64-bit memory accesses in i586 are atomic.  By using an SSE
register we can make such memory accesses in one instruction without
bus-locking.  Need to compile with -msse (or higher) to enable this
feature.

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