All users which relied on the default constructor use a None lock rank
for now. This will make it easier to in the future remove LockRank and
actually annotate the ranks by searching for None.
The code in Spinlock.h has no architectural specific logic, thus can be
moved to the Arch directory. This contains no functional change.
Also add the Spinlock.cpp file for aarch64 which contains stubs for the
lock and unlock functions.
Spinlocks are tied to the platform they are built for, this is why they
have been moved into the Arch folder. They are still available via
"Locking/Spinlock.h"
An Aarch64 stub has been created
