xtensa: clean up inline assembly in futex.h

Replace numeric inline assembly parameters with named parameters.
Drop unused parameters from the futex_atomic_cmpxchg_inatomic.
Use new temporary variable to hold target address in the fixup code in
futex_atomic_cmpxchg_inatomic. Conditionalize function bodies so that
only 'return -ENOSYS' is left in configurations without futex support.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>