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

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, 267401
Reviewed by:	dim
MFC after:	1 week
Obtained from:	LLVM commit 52516782972730ff065a34123a9d8876da08c254
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D37268

(cherry picked from commit 9c231325e77b1989c006daf083a3e9bd143c3d07)