alpha: replace hardcoded stack offsets with autogenerated ones

This allows the assembly in entry.S to automatically keep in sync with
changes in the stack layout (struct pt_regs and struct switch_stack).

Cc: stable@vger.kernel.org
Tested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Tested-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Ivan Kokshaysky <ink@unseen.parts>
Signed-off-by: Matt Turner <mattst88@gmail.com>