arch-gcn3: implement multi-dword buffer loads and stores

arch-gcn3: implement multi-dword buffer loads and stores

Add support for all multi-dword buffer loads and stores:
buffer_load_dword x2, x3, and x4 and buffer_store_dword x2, x3, and x4

Change-Id: I4017b6b4f625fc92002ce8ade695ae29700fa55e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29946
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>