LinuxKPI: add get_random_u8()

Add a get_random_u8() implementation following the u36 and u64 versions.
We'll likely want to macro-ify them in the future and add all the types
which makes sense just to be done.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D46464