x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

CVE-2017-5715 (Spectre v2 Intel)

There are 2 ways to control IBPB and IBRS

1. At boot time
	noibrs kernel boot parameter will disable IBRS usage
	noibpb kernel boot parameter will disable IBPB usage
Otherwise if the above parameters are not specified, the system
will enable ibrs and ibpb usage if the cpu supports it.

2. At run time
	echo 0 > /proc/sys/kernel/ibrs_enabled will turn off IBRS
	echo 1 > /proc/sys/kernel/ibrs_enabled will turn on IBRS in kernel
	echo 2 > /proc/sys/kernel/ibrs_enabled will turn on IBRS in both userspace and kernel

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
[marcelo.cerri@canonical.com: add x86 guards to kernel/smp.c]
[marcelo.cerri@canonical.com: include asm/msr.h under x86 guard in kernel/sysctl.c]
Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>