arch: cpu: Track kernel stats using the base ISA agnostic type.

Then cast to the ISA specific type when necessary. This removes
(mostly) an ISA specific aspect to some of the interfaces. The ISA
specific version of the kernel stats still needs to be constructed and
stored in a few places which means that kernel_stats.hh still needs to
be a switching arch header, for instance.

In the future, I'd like to make the kernel its own object like the
Process objects in SE mode, and then it would be able to instantiate
and maintain its own stats.

Change-Id: I8309d49019124f6bea1482aaea5b5b34e8c97433
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18429
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

alpha: Stop using architecture specific register types.

Change-Id: I4052000014c9f6f9ecefd3f37e58595c61599484
Reviewed-on: https://gem5-review.googlesource.com/c/14461
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

style: [patch 1/22] use /r/3648/ to reorganize includes

style: Make a style pass over the whole arch/alpha directory.

alpha: Clean up namespace usage.

arch: TheISA shouldn't really ever be used in the arch directory.
We should always refer to the specific ISA in that arch directory.
This is especially necessary if we're ever going to make it to the
point where we actually have heterogeneous systems.

*MiscReg->*MiscRegNoEffect, *MiscRegWithEffect->*MiscReg

Put kernel_stats back into arch.