This patch modifies the Coalescer and VIPER protocol to support memory synchronization requests and write-completion responses that are required by upcoming GCN3 implementation. VIPER protocol is simplified to be a solely write-through protocol. Change-Id: Iccfa3d749a0301172a1cc567c59609bb548dace6 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29913 Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com> Reviewed-by: Jason Lowe-Power <power.jg@gmail.com> Reviewed-by: Bradford Beckmann <brad.beckmann@amd.com> Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com> Maintainer: Bradford Beckmann <brad.beckmann@amd.com> Tested-by: kokoro <noreply+kokoro@google.com>