[X86][CET] Add Diags for targets pre to i686 for `-fcf-protection`

Intel Control-flow Enforcement Technology (CET) provides new instructions `endbr32/64` for the indirect branch control. They are NOPs on i686 and new targets. We need to check for that in case it crashes on older targets.

Fixes #58737

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D137317