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>

lib/ovs-atomic.h

@@ -248,10 +248,10 @@
     #if __CHECKER__
         /* sparse doesn't understand some GCC extensions we use. */
         #include "ovs-atomic-pthreads.h"
-    #elif HAVE_STDATOMIC_H
-        #include "ovs-atomic-c11.h"
     #elif __has_extension(c_atomic)
         #include "ovs-atomic-clang.h"
+    #elif HAVE_STDATOMIC_H
+        #include "ovs-atomic-c11.h"
     #elif __GNUC__ >= 4 && __GNUC_MINOR__ >= 7
         #include "ovs-atomic-gcc4.7+.h"
     #elif HAVE_GCC4_ATOMICS