sys: move SAN and COVERAGE options handling to kern.mk

[khng]

Summary

This allows the flags to be picked up more easily when building external
modules.

Sponsored by: Juniper Networks, Inc.

[markj]

I think there's some overlap here with D45498. At a glance this seems fine to me.

[khng]

Updated to adapt to NOSAN's introduction

[markj]

This seems ok, but the kernel build is complicated and it's hard to be confident in such changes just by reading the diff. Please make sure you test building of all of the *SAN kernels (amd64 and arm64) and look at the build output to verify that -fsanitizer is being specified both for the kernel and for kernel modules.

[khng]

Remove duplicated fsanitize flags during module build