x86/speculation: Remove unnecessary retpoline alternatives

x86/speculation: Remove unnecessary retpoline alternatives

Now that the X86_FEATURE_RETPOLINE is always set, some assembly
alternatives can be simplied or even removed. Also add early
check of retpoline_enabled_key in CALL_NOSPEC to avoid an extra
call into thunk when retpoline is disabled.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
(cherry picked from UEK5 commit eb88d822befdc73952ae7c00cfcbce9ff5aad574)

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>