x86/bugs: Restructure spectre_v2 mitigation

Restructure spectre_v2 to use select/update/apply functions to create
consistent vulnerability handling.

The spectre_v2 mitigation may be updated based on the selected retbleed
mitigation.

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/20250418161721.1855190-14-david.kaplan@amd.com