w32: Fix configure test for -march=i486

The latest version of MinGW needs a test for __sync_val_compare_and_swap
to fix a missing symbol linker error.
Reported-by: NIgor Mitsyanko <i.mitsyanko@gmail.com>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Message-id: 1368301619-32097-2-git-send-email-sw@weilnetz.de
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -1346,6 +1346,7 @@ static int sfaa(int *ptr)
 int main(void)
 {
   int val = 42;
+  val = __sync_val_compare_and_swap(&val, 0, 1);
   sfaa(&val);
   return val;
 }