sysfs/cpu: Add vulnerability folder

sysfs/cpu: Add vulnerability folder

As the meltdown/spectre problem affects several CPU architectures, it makes
sense to have common way to express whether a system is affected by a
particular vulnerability or not. If affected the way to express the
mitigation should be common as well.

Create /sys/devices/system/cpu/vulnerabilities folder and files for
meltdown, spectre_v1 and spectre_v2.

Allow architectures to override the show function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lkml.kernel.org/r/20180107214913.096657732@linutronix.de
(cherry picked from commit 87590ce6e373d1a5401f6539f0c59ef92dd924a9)
Orabug: 27353383
Signed-off-by: Kanth Ghatraju <kanth.ghatraju@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Conflicts:
	Documentation/ABI/testing/sysfs-devices-system-cpu
	include/linux/cpu.h

	Conflicts resolved by picking only the changes from original
	patch.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>