arch-gcn3: Implementation of s_sleep

arch-gcn3: Implementation of s_sleep

This changeset implements the s_sleep instruction in a similar
way to s_waitcnt.

Change-Id: I4811c318ac2c76c485e2bfd9d93baa1205ecf183
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39115
Maintainer: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>