alpha: irq: run all handlers with interrupts disabled

Linux has expected that interrupt handlers are executed with local
interrupts disabled for a while now, so ensure that this is the case on
Alpha even for non-device interrupts such as IPIs.

Without this patch, secondary boot results in the following backtrace:

  warning: at kernel/softirq.c:139 __local_bh_enable+0xb8/0xd0()
  trace:
    __local_bh_enable+0xb8/0xd0
    irq_enter+0x74/0xa0
    scheduler_ipi+0x50/0x100
    handle_ipi+0x84/0x260
    do_entint+0x1ac/0x2e0
    irq_exit+0x60/0xa0
    handle_irq+0x98/0x100
    do_entint+0x2c8/0x2e0
    ret_from_sys_call+0x0/0x10
    load_balance+0x3e4/0x870
    cpu_idle+0x24/0x80
    rcu_eqs_enter_common.isra.38+0x0/0x120
    cpu_idle+0x40/0x80
    rest_init+0xc0/0xe0
    _stext+0x1c/0x20

A similar dump occurs if you try to reboot using magic-sysrq.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>