asm/rwonce: Allow __READ_ONCE to be overridden by the architecture

The meat and potatoes of READ_ONCE() is defined by the __READ_ONCE()
macro, which uses a volatile casts in an attempt to avoid tearing of
byte, halfword, word and double-word accesses. Allow this to be
overridden by the architecture code in the case that things like memory
barriers are also required.

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