alpha: Select ARCH_WANT_IRQS_OFF_ACTIVATE_MM

activate_mm() on Alpha calls __load_new_mm_context() which assigns a
new ASID and invokes PALcode swpctx. On SMP, an interrupt arriving
between the ASID update and the page table switch could see
inconsistent state. Ensure activate_mm() is called with interrupts
disabled during exec.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Matt Turner <mattst88@gmail.com>