cpu: Consolidate and move the CPU's calls to TheISA::initCPU.

cpu: Consolidate and move the CPU's calls to TheISA::initCPU.

TheISA::initCPU is basically an ISA specific implementation of reset
logic on architectural state. As such, it only needs to be called if
we're not going to load a checkpoint, ie in initState.

Also, since the implementation was the same across all CPUs, this
change collapses all the individual implementations down into the base
CPU class.

Change-Id: Id68133fd7f31619c90bf7b3aad35ae20871acaa4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24189
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>