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
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D39581