Documentation: Add swapgs description to the Spectre v1 documentation

commit 4c92057661a3412f547ede95715641d7ee16ddac upstream.

Add documentation to the Spectre document about the new swapgs variant of
Spectre v1.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 4.9: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>