With the update to llvm 13 we are able to tell the compiler it can find
the SSP canary relative to the register that holds the userspace stack
pointer. As this is unused in most of the kernel it can be used here
to point to a per-thread SSP canary.
As the kernel could be built with an old toolchain, e.g. when upgrading
from 13, add a warning that the options was enabled but the compiler
doesn't support it to both the build and kernel boot.
Discussed with: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33079
ae92ace05f