alpha: Override READ_ONCE() with barriered implementation

Rather then relying on the core code to use smp_read_barrier_depends()
as part of the READ_ONCE() definition, instead override __READ_ONCE()
in the Alpha code so that it generates the required mb() and then
implement smp_load_acquire() using the new macro to avoid redundant
back-to-back barriers from the generic implementation.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>