x86/spectre_v2: Figure out if STUFF_RSB macro needs to be used.

x86/spectre_v2: Figure out if STUFF_RSB macro needs to be used.

If IBRS is on, STUFF_RSB only needs to be enabled if the CPU
does not have SMEP enabled.

We change the macro to depend on a synthetic one called
X86_FEATURE_STUFF_RSB - which will only be turned on
if 'spectre_v2=ibrs' is set and the BIOS does not have SMEP
enabled.

(And naturally also if the kernel is compiled without retpoline
and the CPU does not have SMEP).

Orabug: 27477743
CVE: CVE-2017-5715

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