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.

[ Shile: Resolved the conflicts by adding the missed file ]
Signed-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NShile Zhang <shile.zhang@linux.alibaba.com>
Acked-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

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