ovs-atomic: Prefer Clang intrinsics over <stdatomic.h>.

On my Debian "jessie" system, <stdatomic.h> provided by GCC 4.9 is busted
when Clang 3.5 tries to use it.  Even a trivial program like this:

    #include <stdatomic.h>

    void
    foo(void)
    {
         _Atomic(int) x;
         atomic_fetch_add(&x, 1);
}

yields:

     atomic.c:7:5: error: address argument to atomic operation must be a
        pointer to integer or pointer ('_Atomic(int) *' invalid)

The Clang-specific version of ovs-atomic.h stills works, though, so this
commit works around the problem.

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