arm64: lib: use pair accessors for copy_*_user routines

commit 23e94994464a7281838785675e09c8ed1055f62f upstream.

The AArch64 instruction set contains load/store pair memory accessors,
so use these in our copy_*_user routines to transfer 16 bytes per
iteration.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: David Brown <david.brown@linaro.org>