arch: Move many of the generic files outside an NULL guard.
arch: Move many of the generic files outside an NULL guard.

These files can be compiled successfully even if the ISA is the NULL
ISA.

Change-Id: I67133ea674f678f33b0aa1ef55af719f2869241d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34169
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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