[MIPS] Use nacl_atomic_is_lock_free instead of c11_atomic_is_lock_free

Use nacl_atomic_is_lock_free instead of c11_atomic_is_lock_free, as MIPS
does not have (yet) implementations of these c11 functions for 64-bit types.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4205
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1286693006 .

include/atomic

diff --git a/include/atomic b/include/atomic
index fe85609..c731485 100644
--- a/include/atomic
+++ b/include/atomic
@@ -818,7 +818,7 @@
   // @LOCALMOD-BEGIN The NaCl builtin delays resolution of the lock-free
   //                 property until translation time (time at which the
   //                 actual target is known).
-#if defined (__pnacl__)
+#if defined (__pnacl__) || (defined(__mips__) && defined (__native_client__))
     _LIBCPP_INLINE_VISIBILITY
     bool is_lock_free() const volatile _NOEXCEPT
         {return __nacl_atomic_is_lock_free(sizeof(_Tp), &__a_);}