Clang: 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.

PR: 264497
Reviewed by: dim
MFC after: 1 week
Obtained from: LLVM commit 52516782972730ff065a34123a9d8876da08c254
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37268