x86/speculation: Retpoline should always be available on Skylake

x86/speculation: Retpoline should always be available on Skylake

Now that we can dynamically toggle retpoline on or off, retpoline
should always be available even on Skylake platforms, without
having to explicitly select retpoline at boot time.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Jamie Iles <jamie.iles@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
(cherry picked from UEK5 commit 7bbc586ac477df24012552712df10ab9ae300919)

Orabug: 28801831

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Jamie Iles <jamie.iles@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>