ovs-atomic: Add native Clang implementation.

With this implementation I get warnings with Clang on GNU/Linux when the
previous patch is not applied.  This ought to make it easier to avoid
introducing new problems in the future even without building on FreeBSD.

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