arch-riscv: Fix disassembling for atomic instructions

arch-riscv: Fix disassembling for atomic instructions

The original Gem5 does not give correct disassembly for atomic
instructions, which are implemented with one or two micro instructions.
The correct register indices are not decoded until subsequent micro
instruction is processed. This patch fixes the problem by getting the
register indices and other properties (aq and rl) from certain bitfields
of the machine code in the disassembling function.

Change-Id: I2cdaf0b3c48ff266f19ca707a5de48c9050b3897
Signed-off-by: Ian Jiang <ianjiang.ict@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22568
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Alec Roelke <alec.roelke@gmail.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>