arch: Stop using switching header files in ISA specific files.

arch: Stop using switching header files in ISA specific files.

We know what ISA we want, we don't need to use the indirection.

Change-Id: I57eb2737bb4d9abb562b857ad2c3238c641199d2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40104
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>