base: Get rid of the unused global pointer in object files.

base: Get rid of the unused global pointer in object files.

This was only ever read from Alpha, and nothing ever set it.
It defaulted to zero, so this change just propogates that value through
to the Alpha Process class.

Change-Id: I569cf9d61a37322dbd88de1038a2af74c64bbe7a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21461
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>