x86/spectre: Favor IBRS on Skylake over retpoline

x86/spectre: Favor IBRS on Skylake over retpoline

Couple of rules around this. If the user has choosen:

 spectre_v2=retpoline
 spectre_v2=retpoline,generic

That we will respect their wishes.

If the customer has:

 spectre_v2=auto (by default)
 spectre_v2=force

Then we will figure out if this is a machine with Skylake
affected CPUS. If so, we will pick IBRS over retpoline
if IBRS is available.

And lastly, if the kernel is compiled without retpoline
support we will pick IBRS over minimal retpoline support
(if IBRS is available).

In other words the priority for non-Skylake is:

retpoline
IBRS
minimal asm

On Skylake:

IBRS
retpoline
minimal asm

Orabug: 27477743
CVE: CVE-2017-5715

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>