x86/srso: Add IBPB

x86/srso: Add IBPB

Add the option to mitigate using IBPB on a kernel entry. Pull in the
Retbleed alternative so that the IBPB call from there can be used. Also,
if Retbleed mitigation is done using IBPB, the same mitigation can and
must be used here.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
CVE-2023-20569
(backported from commit 233d6f68b98d480a7c42ebe78c38f79d44741ca9)
[cascardo: conflict in ANNOTATE_UNRET_END vs VALIDATE_UNRET_END]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>