Make the arm64 get_pcpu a function again
We assume the pointer returned from get_pcpu will be consistent even
if the thread is moved to a new CPU. Fix this by partially reverting
63c858a04d565 to make get_pcpu a function again.
Sponsored by: The FreeBSD Foundation
(cherry picked from commit ed306634266002a05c88c3a4cd036c99d4cd139a)