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

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>

Bug: 31432001
Change-Id: I383662ba12fc77a435011821ab666a52f30efe3a
(cherry picked from commit 23e94994)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>