arm64: Update copy_to_user()

As with its counterpart, replace copy_to_user() with a new and improved
version similarly derived from memcpy(). Different tradeoffs from the
base implementation are made relative to copy_from_user() to get the
most consistent results across different microarchitectures, but the
overall shape of the performance gain ends up about the same.

The exception fixups are even more comical this time around, but that's
down to now needing to reconstruct the faulting address, and cope with
overlapping stores at various points. Again, improvements to the
exception mechanism could significantly simplify things in future.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>