arch-gcn3: Remove gcn3 (#781)

Related to issue #703 , this PR removes GCN3 related files and updates
source code, documentation, and tests to switch over to Vega is that was
not done already. Highlights are:

 - Remove all src/arch/amdgpu/gcn3 files and update Kconfigs.
 - Remove references to GCN3 and replace with Vega where applicable.
- Update the build targets in the gcn-gpu Docker. This will need to be
rebuilt but not urgently.
- Remove the GCN3 tag in testlib. Most tests seem to be using Vega
already, so that commit is small.

src/dev/amdgpu/hwreg_defines.hh

@@ -44,7 +44,7 @@ namespace gem5
 
 /*
  * Further descriptions can be found in the "Hardware Register Values" table
- * in any of the GCN3, Vega, CDNA1, CNDA2, or RDNA ISA manuals.
+ * in any of the Vega, CDNA, or RDNA ISA manuals.
  */
 enum amdgpu_hwreg
 {