Kernel: Zero out .bss contents on aarch64

[nico]

After building and running

 objcopy -O binary Build/aarch64/Kernel/Prekernel/Prekernel \
                   /media/sdcard/kernel8.img

things start booting on an actual RPi4 :^)

(Assuming the sdcard contains RPi firmware, an empty config.txt,
and no other kernel*.img files).

[nico]

This is now at RPi4-bare-metal "Hello world" level. It's something.

Hangs either querying firmware revision or printing it, but having any output at all means printf* debugging is yes.

(*: No actual printf yet.)

[nico]

Now it prints Firmware version: 0. That's not right either, but at least it's more clear it doesn't hang. (It printed the \r\n before, so if you looked closely it was clear before too.)