early-console: implement arch independent early console

includes an implementation of arch_early_console for x64
using isa-serial, and an implementation for aarch64
using the (for now write-only) pl011.

This restores output functionality for AArch64 lost in
commit 9bbbe9dc.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>