cpu: Don't construct and then copy the decoder in SimpleThread.

cpu: Don't construct and then copy the decoder in SimpleThread.

The SimpleThread constructor was constructing a temporary copy of the
decoder, and then copying it into it's local version. This copy is a
waste, and also requires there to be a copy operator for the Decoder.

Change-Id: I1123b4ec767e08ceb2f108b3a6b19ca18d7c677c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32900
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>