softlockup: make asm/irq_regs.h available on every platform

The softlockup detector would like to use get_irq_regs(), so generalize the
availability on every Linux architecture.

(It is fine for an architecture to always return NULL to get_irq_regs(),
which it does by default.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Ian Molton <spyro@f2s.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Mikael Starvik <starvik@axis.com>
Cc: Miles Bader <uclinux-v850@lsi.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>