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>