arch-riscv: Fix compressed branch op offset

arch-riscv: Fix compressed branch op offset

There is a bug in RISC-V's compressed branch instructions where the
offsets are not stored in ImmOp's immediate field, causing incorrect
branchTarget() return values.  This patch adds a new compressed branch
op format, CBOp, which correctly stores the offset.

Change-Id: Iac6e9b091d63f3dce4717ee5a9ec31a7cbd6c377
Reviewed-on: https://gem5-review.googlesource.com/8441
Reviewed-by: Tuan Ta <qtt2@cornell.edu>
Maintainer: Alec Roelke <ar4jc@virginia.edu>