x86/bugs: Restructure ITS mitigation

Restructure the ITS mitigation to use select/update/apply functions like
the other mitigations.

There is a particularly complex interaction between ITS and Retbleed as CDT
(Call Depth Tracking) is a mitigation for both, and either its=stuff or
retbleed=stuff will attempt to enable CDT.

retbleed_update_mitigation() runs first and will check the necessary
pre-conditions for CDT if either ITS or Retbleed stuffing is selected.  If
checks pass and ITS stuffing is selected, it will select stuffing for
Retbleed as well.

its_update_mitigation() runs after and will either select stuffing if
retbleed stuffing was enabled, or fall back to the default (aligned thunks)
if stuffing could not be enabled.

Enablement of CDT is done exclusively in retbleed_apply_mitigation().
its_apply_mitigation() is only used to enable aligned thunks.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/20250516193212.128782-1-david.kaplan@amd.com