ARM: locks: prefetch the destination word for write prior to strex

The cost of changing a cacheline from shared to exclusive state can be
significant, especially when this is triggered by an exclusive store,
since it may result in having to retry the transaction.

This patch prefixes our {spin,read,write}_[try]lock implementations with
pldw instructions (on CPUs which support them) to try and grab the line
in exclusive state from the start. arch_rwlock_t is changed to avoid
using a volatile member, since this generates compiler warnings when
falling back on the __builtin_prefetch intrinsic which expects a const
void * argument.

Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>