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.

Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ed306634266002a05c88c3a4cd036c99d4cd139a)
(cherry picked from commit 1169099f8fd981f2e085f451f0b4a62e18bfe507)