Instead of having two separate implementations of AK::RefCounted, one
for userspace and one for kernelspace, there is now RefCounted and
AtomicRefCounted.
This will somwhat help unify them also under the same SysFS directory in
the commit.
Also, it feels much more like this change reflects the reality that both
ACPI and the BIOS are part of the firmware on x86 computers.