gpu-compute,misc: Fix Clang missing override errors

gpu-compute,misc: Fix Clang missing override errors

Clang fails to compile GCN3 due to missing overrides in
`src/gpu-compute/gpu_command_processor.hh`. This commit fixes this
errror.

Change-Id: I6da9fce7c3eb86a5418a931ee4f225cceda488a5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40396
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>