cpu: Rename GpuThread in gpu_ruby_test tester

The GpuThread class will be used as an abstract class for any thread
type (CPU, GPU, DMA) therefore changing to a more appropriate name.

Change-Id: If241edb53ea405c95b0315c609176c6470b29931
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39935
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
16 files changed