riscv: support for Cache-Block Operations (CBO).

CBO represents a subset of Cache-Management Operations (CMO) spec.
While the CMO spec encompasses all operations on caches, the CBO subset
operates on cache blocks only.

Detect Zicbom, Zicboz and Zicbop extensions and provide cache invalidation
handlers based on Zicbom instructions.

Sponsored by: UKRI
Differential Revision: https://reviews.freebsd.org/D49852