Add AUTO_VAR_INIT to ensure the stack it initialised

[andrew]

Summary

AUTO_VAR_INIT adds a flag to clang to ensure the stack is initialised.
It will write a known value to the stack before it is used to ensure
no previous stack memory is leaked across functions.

This is currently disabled by default on amd64 as it seems to call memset
before ifunc resolution has occurred.

On a 4 core arm64 CPU no significant difference in kernel build sys and
real timd and only a 0.06% increase in user time was observed when this
was enabled.

[emaste]

sys/conf/kern.pre.mk

149

At some point we need a better way to handle this for all such options

[kib]

I would prefer that we do not triggered this option at all, this is no longer a C language.

I am not even sure is this a useful debugging technique or not.

[andrew]

It's not intended to be a debugging tool, it's to stop leaking from kernel stack, e.g. CVE-2019-15875 https://lists.freebsd.org/pipermail/freebsd-announce/2020-January/001926.html

The llvm review that added the option has more details on how it works: https://reviews.llvm.org/D54604

[emaste]

See also PR251083