[gcc r12-4619] x86: Document -fcf-protection requires i686 or newer

[H.J. Lu]

https://gcc.gnu.org/g:1373066a46d8d47abd97e46a005aef3b3dbfe94a

commit r12-4619-g1373066a46d8d47abd97e46a005aef3b3dbfe94a
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Oct 21 09:45:14 2021 -0700

    x86: Document -fcf-protection requires i686 or newer
    
            PR target/98667
            * doc/invoke.texi: Document -fcf-protection requires i686 or
            new.

Diff:
---
 gcc/doc/invoke.texi | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index c66a25fcd69..71992b8c597 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -15542,7 +15542,8 @@ which functions and calls should be skipped from instrumentation
 (@pxref{Function Attributes}).
 
 Currently the x86 GNU/Linux target provides an implementation based
-on Intel Control-flow Enforcement Technology (CET).
+on Intel Control-flow Enforcement Technology (CET) which works for
+i686 processor or newer.
 
 @item -fstack-protector
 @opindex fstack-protector