ARM: 8390/1: irqflags: Get arch_irqs_disabled from asm-generic

Commit cb1293e2

 ("ARM: 8375/1: disable some options on ARMv7-M")
causes the build to on ARMv7-M machines:

  CC      arch/arm/kernel/asm-offsets.s
In file included from include/linux/sem.h:5:0,
                 from include/linux/sched.h:35,
                 from arch/arm/kernel/asm-offsets.c:14:
include/linux/rcupdate.h: In function 'rcu_read_lock_sched_held':
include/linux/rcupdate.h:539:2: error: implicit declaration of function
'arch_irqs_disabled' [-Werror=implicit-function-declaration]
  return preempt_count() != 0 || irqs_disabled();

asm-generic/irqflags.h provides an implementation of arch_irqs_disabled().
Lets grab an implementation from there!

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>