arch-riscv: Fix WFI for O3 CPU

This commit:
https://gem5-review.googlesource.com/c/public/gem5/+/61511
introduced a bug where the O3 CPU hangs. This is because WFI must be
tagged as `IsNonSpeculative`, `IsQuiesce`, and `IsSerializeAfter` to
function correctly with O3 CPUs.

Change-Id: I8b6cb049710d05f37f89a9ce22acc604112bc445
Issue-on: https://gem5.atlassian.net/browse/GEM5-1323
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70657
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Roger Chang <rogerycchang@google.com>
Reviewed-by: Jui-min Lee <fcrh@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/riscv/isa/decoder.isa

@@ -2278,7 +2278,8 @@ decode QUADRANT default Unknown::unknown() {
                                 && xc->readMiscReg(MISCREG_NMIP) == 0) {
                                 tc->quiesce();
                             }
-                        }}, No_OpClass);
+                        }}, IsNonSpeculative, IsQuiesce,
+                            IsSerializeAfter, No_OpClass);
                     }
                     0x9: sfence_vma({{
                         STATUS status = xc->readMiscReg(MISCREG_STATUS);