x86/bugs: Add attack vector controls for SSB

Attack vector controls for SSB were missed in the initial attack vector series.
The default mitigation for SSB requires user-space opt-in so it is only
relevant for user->user attacks.  Check with attack vector controls when
the command is auto - i.e., no explicit user selection has been done.

Fixes: 2d31d287 ("x86/bugs: Define attack vectors relevant for each bug")
Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250819192200.2003074-5-david.kaplan@amd.com