arch: alpha: Fix an 8 year old bug from the transition to pc objects.

arch: alpha: Fix an 8 year old bug from the transition to pc objects.

When we transitioned from having PCs and nextPCs stored as individual items
in the ThreadContext and went to having PC objects with abstracted
components (micro PCs, delay slots, variable instruction lengths, etc.), a
small mistake was made when translating the alpha StackTrace code.

Change-Id: Ib5dd65c53a26920e66899f421902607efd139e71
Reviewed-on: https://gem5-review.googlesource.com/9702
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>