arch-vega: Add Vega ISA as a copy of GCN3

This changeset adds Vega support as a copy of GCN3.
Configs have been modified to include both ISAs.
Current implementation is not complete and needs
modifications to fully comply with the ISA manual:

https://developer.amd.com/wp-content/resources/
Vega_Shader_ISA_28July2017.pdf

Change-Id: I608aa6747a45594f8e1bd7802da1883cf612168b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42204
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>

MAINTAINERS.yaml

@@ -68,6 +68,12 @@ arch-gcn3:
     - Matt Poremba <matthew.poremba@amd.com>
     - Matt Sinclair <sinclair@cs.wisc.edu>
 
+arch-vega:
+  status: maintained
+  maintainers:
+    - Matt Poremba <matthew.poremba@amd.com>
+    - Matt Sinclair <sinclair@cs.wisc.edu>
+
 arch-mips:
   status: orphaned