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.
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