kern: Globally define abs64

So we can use it in non-linuxkpi sources.

Reviewed by: emaste, mmel
Differential Revision: https://reviews.freebsd.org/D41767
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 229c65a83fbe0ede8c617e35d8f8c14d5ebadc19)