ovs-atomic: New functions atomic_flag_init(), atomic_flag_destroy().

ovs-atomic: New functions atomic_flag_init(), atomic_flag_destroy().

Standard C11 doesn't need these functions because it is able to require
implementations not to need them.  But we can't construct a portable
implementation that does not need them in every case, so this commit adds
them.

These functions are only needed for atomic_flag objects that are
dynamically allocated (because statically allocated objects can use
ATOMIC_FLAG_INIT).  So far there aren't any of those, but an upcoming
commit will introduce one.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>