alpha: make thread_saved_pc static

The only user of thread_saved_pc() in non-arch-specific code was removed
in commit 8243d559 ("sched/core: Remove pointless printout in
sched_show_task()"), so it no longer needs to be globally defined for
Alpha and can be made static.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Matt Turner <mattst88@gmail.com>