cpu: Ignore "mitigations" kernel parameter if CPU_MITIGATIONS=n

Explicitly disallow enabling mitigations at runtime for kernels that were
built with CONFIG_CPU_MITIGATIONS=n, as some architectures may omit code
entirely if mitigations are disabled at compile time.

E.g. on x86, a large pile of Kconfigs are buried behind CPU_MITIGATIONS,
and trying to provide sane behavior for retroactively enabling mitigations
is extremely difficult, bordering on impossible.  E.g. page table isolation
and call depth tracking require build-time support, BHI mitigations will
still be off without additional kernel parameters, etc.

  [ bp: Touchups. ]

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240420000556.2645001-3-seanjc@google.com