arch-riscv: Add new misa bit union

The new misa bit union type can help get and set misa CSR more
clearily

Change-Id: Id48b140968a0e8021b09782815aa612b409ac75b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/68917
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
2 files changed