Documentation: Add section about CPU vulnerabilities for Spectre

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: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/hw-vuln/index.rst

@@ -9,5 +9,6 @@ are configurable at compile, boot or run time.
 .. toctree::
    :maxdepth: 1
 
+   spectre
    l1tf
    mds

Documentation/userspace-api/spec_ctrl.rst

@@ -49,6 +49,8 @@ If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is
 available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
 misfeature will fail.
 
+.. _set_spec_ctrl:
+
 PR_SET_SPECULATION_CTRL
 -----------------------