x86/bugs: Do not enable IBPB-on-entry when IBPB is not supported

There are some VM configurations which have Skylake model but do not
support IBPB. In those cases, when using retbleed=ibpb, userspace is going
to be killed and kernel is going to panic.

If the CPU does not support IBPB, warn and proceed with the auto option. Also,
do not fallback to IBPB on AMD/Hygon systems if it is not supported.

Fixes: 3ebc1700

 ("x86/bugs: Add retbleed=ibpb")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Borislav Petkov <bp@suse.de>