PORTING: add information about the AArch64 port

Signed-off-by: Jani Kokkonen <jani.kokkonen@huawei.com>
Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
arch/aarch64: starting point for the AArch64 port

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Jani Kokkonen <jani.kokkonen@huawei.com>
libc: add simple memcpy_base_backwards

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>