[Bug target/81708] The x86 stack canary location should be customizable

[luto at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708

--- Comment #2 from Andy Lutomirski <luto at kernel dot org> ---
(In reply to H. Peter Anvin from comment #1)
> Well, you can choose between "__stack_chk_guard(%rip)" and "%gs:40"... :)

Wow, I guess I didn't even consider the former.  That would be option 5:
symbol(%rip).  Let's not use it for the kernel.

For x86_32, we want %fs:symbol.  %fs:constant would be tolarable, too.  For
x86_64, it depends on the end game for percpu access, I suppose.