KASAN: add bus_space*read*_8 for aarch64

Add the remaining bus_space*read*_8 functions conditionally for
only arm64 in order to not break KASAN builds with new code using
one of them.

Suggested by: markj
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D39581

(cherry picked from commit 42742fe725102a73b09800f4d041ad69b5d81edb)