[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 .