sparc: Introduce constants for pseudo integer registers.

sparc: Introduce constants for pseudo integer registers.

These are "integer" registers which are renamed, but which aren't
normally considered integer registers by the ISA. They had been indexed
by adding an opaque constant to the number of official integer
registers which obscured what they were, and was also fragile and
invited mistakes.

Change-Id: Idab8cf4d889682b98c7c81a00d9a92d8e3bb3a05
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23445
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>