lttng-statedump-impl: Use generic hard irqs for Linux >= 3.12

lttng-statedump-impl: Use generic hard irqs for Linux >= 3.12

Quoting the original patch changelog from Otavio Salvador:

> The Linux kernel 3.12 uses the generic hard irqs system for all
> architectures and dropped the GENERIC_HARDIRQ option, as can be seen
> at the commit quoted below:
>
> ,----
> | commit 0244ad004a54e39308d495fee0a2e637f8b5c317
> | Author: Martin Schwidefsky <schwidefsky@de.ibm.com>
> | Date:   Fri Aug 30 09:39:53 2013 +0200
> |
> |     Remove GENERIC_HARDIRQ config option
> |
> |     After the last architecture switched to generic hard irqs the config
> |     options HAVE_GENERIC_HARDIRQS & GENERIC_HARDIRQS and the related code
> |     for !CONFIG_GENERIC_HARDIRQS can be removed.
> |
> |     Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> `----

Introduce wrapper/irq.h to move the feature availability testing logic
into a specific wrapper header. It now tests if the kernel version is
>= 3.12 or if CONFIG_GENERIC_HARDIRQS is defined (for older kernels).
Introduce the lttng-specific CONFIG_LTTNG_HAS_LIST_IRQ to track
availability of this feature within LTTng.

Reported-by: Philippe Mangaud <r49081@freescale.com>
Reported-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>