objtool: Validate kCFI calls

Validate that all indirect calls adhere to kCFI rules. Notably doing
nocfi indirect call to a cfi function is broken.

Apparently some Rust 'core' code violates this and explodes when ran
with FineIBT.

All the ANNOTATE_NOCFI_SYM sites are prime targets for attackers.

 - runtime EFI is especially henous because it also needs to disable
   IBT. Basically calling unknown code without CFI protection at
   runtime is a massice security issue.

 - Kexec image handover; if you can exploit this, you get to keep it :-)

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lkml.kernel.org/r/20250714103441.496787279@infradead.org

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