Add documentation for Spectre vulnerability and the mitigation mechanisms:

- Explain the problem and risks
- Document the mitigation mechanisms
- Document the command line controls
- Document the sysfs files
Co-developed-by: NAndi Kleen <ak@linux.intel.com>
Signed-off-by: NAndi Kleen <ak@linux.intel.com>
Co-developed-by: NTim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: NTim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: NRandy Dunlap <rdunlap@infradead.org>
Reviewed-by: NThomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: NJonathan Corbet <corbet@lwn.net>