Documentation: Add swapgs description to the Spectre v1 documentation

commit 4c92057 upstream.

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

[ Shile: Resolved the conflicts by adding the missed file ]

Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Shile Zhang <[email protected]>
Acked-by: Joseph Qi <[email protected]>

Conflicts:
	Documentation/admin-guide/hw-vuln/spectre.rst