x86/spectre_v2: Add VMEXIT_FILL_RSB instead of RETPOLINE

x86/spectre_v2: Add VMEXIT_FILL_RSB instead of RETPOLINE

The backport of "x86/retpoline: Fill return stack buffer on vmexit"
made the full stuffing of RSB only enabled if the kernel had
selected X86_FEATURE_RETPOLINE.

But if we are using IBRS we still want the full RSB stuffing
as it was prior to the backport.

Since we have both retpoline and ibrs wanting it we introduce
a new feature to enable the common mitigation that both of them
need.

Orabug: 27477743
CVE: CVE-2017-5715

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