Use -fdebug-prefix-map in the kernel build.

[markj]

Summary

Our debug info currently contains a few magic paths that are
relative to the object directory. This confuses consumers of kernel
debug info, in particular, syzkaller's code coverage report generation.
In the past I used to frequently see kgdb complain about not being to
find headers under machine/.

clang and modern gcc support -fdebug-map-prefix=src=dst, which tells the
compiler to substitute dst for src in debug info when compiling files
whose path is prefixed by src. This helps with a couple of things:

• The machine, x86 and MACHINE_CPUARCH symlinks. We can just map those to their canonical paths in the src tree, so e.g., machine/frame.h is translated to sys/amd64/include/frame.h.
• Autogenerated headers, like vnode_if.h. We include them via a relative path (just "./") now, so substituting the object directory path gives a better chance of finding them.

TODO: add guards for gcc 4.2.

Test Plan

I verified that this works with in-tree clang and the amd64-gcc compiler from ports.

[markj]

Ed pointed out that we may not want absolute paths in general. That's probably true, but I'll note that this change just makes things consistent for now. I think mapping the symlink paths is a no-brainer, at least.

[emaste]

Yes, as long as the main paths are absolute I think it's no worse to make these changes.

[jhb]

I'm not a huge fan of the OBJDIR one, but I don't see a better alternative. Definitely a fan of the symlink changes. Shame the code is duplicated in two places. It might make sense to have some kind of sys/conf/kern.common.mk or something that has shared logic between kmod.mk and kern.*.mk if that is feasible.

[markj]

• Add compiler version guards.