Support LSE atomics in the arm64 casue* functions

As with atomic(9) use the ARMv8.1 Large System Extension atomic
instructions to implement the userspace compare and swap functions.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35234

(cherry picked from commit 88ac318759f28f34aa991ecc6f336326ec00c501)