mmu: implement mmu for aarch64 with refactoring of x64

Signed-off-by: Jani Kokkonen <jani.kokkonen@huawei.com>
[claudio: some fixes]
Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
aarch64: remove most AARCH64_PORT_STUB related to MMU

also enable core/pagecache.cc in the AArch64 build.

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