x86/bugs: Restructure SSB mitigation

Restructure SSB to use select/apply functions to create consistent
vulnerability handling.

Remove __ssb_select_mitigation() and split the functionality between the
select/apply functions.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/20250418161721.1855190-15-david.kaplan@amd.com