[MIPS] Update/fix futex assembly

    
 o Implement futex_atomic_op_inuser() operation
 o Don't use the R10000-ll/sc bug workaround version for every processor.
   branch likely is deprecated and some historic ll/sc processors don't
   implement it.  In any case it's slow.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>