x86/speculation: Make enhanced IBRS the default spectre v2 mitigation

x86/speculation: Make enhanced IBRS the default spectre v2 mitigation

Currently we use retpoline as the default spectre v2 mitigation.
On future processors that support the capability, enhanced IBRS
will be the default, and otherwise retpoline will be used.

From the upstream patch at:
https://lore.kernel.org/lkml/1533148945-24095-1-git-send-email-sai.praneeth.prakhya@intel.com/

"The reason why Enhanced IBRS is the recommended mitigation on
processors which support it is that these processors also support
CET which provides a defense against ROP attacks. Retpoline is
very similar to ROP techniques and might trigger false positives
in the CET defense."

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Co-developed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
(cherry picked from commit 79bb6288902479281622b4ba0d6723d45732a2cc from UEK5)

Orabug: 28474851

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	arch/x86/kernel/cpu/bugs.c
(In UEK4, the relevant code is in arch/x86/kernel/cpu/bugs_64.c)

Signed-off-by: Brian Maly <brian.maly@oracle.com>