Clang: Add Diags for targets pre to i686 for -fcf-protection

[emaste]

Summary

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.

PR:             264497
Obtained from:  LLVM commit 52516782972730ff065a34123a9d8876da08c254

(I think this will introduce a (trivially resolved) conflict on the next import.)

[dim]

LGTM, should probably be MFC'd too?