arch-riscv: make uret,sret,mret SerializeAfter,NonSpeculative.

arch-riscv: make uret,sret,mret SerializeAfter,NonSpeculative.

These instructions potentially change the privilege level, upon which
the execution of other instructions depends. For that reason, we need to
make *ret SerializeAfter and we also don't want to execute them
speculatively.

Change-Id: If3b5ba6ec3b4102d53c51cf723eba5d5da3eaa2f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25656
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>