x86/speculation: Use static key to enable/disable retpoline

x86/speculation: Use static key to enable/disable retpoline

Change the way retpoline is enabled.

Up to now, the retpoline feature was enabled using alternatives
depending whether the X86_FEATURE_RETPOLINE cpu capability was set
or not. The usage of alternatives prevented enabling or disabling
after the boot sequence.

Now, retpoline is enabled using jump labels controlled by the
retpoline_enabled_key static key. This enables retpoline to be
selectively enabled or disabled even after the boot sequence.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
(cherry picked from UEK5 commit 3d01785866e2a73119ed4e35d5cb6b02edf5957a)

Orabug: 28607548

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>