sanitizers: Avoid building genassym.c and genoffset.c with sanitizers

[markj]

Summary

Some, particularly KASAN, may insert redzones around global symbols,
resulting in incorrect offset definitions because genassym.sh defines
some semantics using symbol sizes.

Ideally I would be able to define this pattern in one place, but I
haven't found a way to define a GENSYM_CFLAGS that actually works for
all of the consumers (kern.post.mk, kmod.mk, sys/conf/files*).

[emaste]

I wonder why amd64 is the only one with a <32>_genassym.o

[olce]

Funny, I stumbled on the genassym/genoffset mechanisms yesterday. FWIW, LGTM.

[markj]

In D42157#961811, @emaste wrote:

I wonder why amd64 is the only one with a <32>_genassym.o

You mean, why doesn't arm64 have one as well? It looks like ia32_genassym defines symbols used by the 32-bit signal trampoline. On arm64, aarch32_sigcode is stuck at the end of locore and uses hex constants instead of actual instructions, so it doesn't need to define SIGF_UC etc.

[emaste]

It would be nice if there was a common place we could define this pattern, but I'm not aware of a clean way to do it. I had the same issue in 0e8b3ab34883efedeeb5e0d3baa90ea98de1e692. Note there are a couple of modules in that change that have their own genassym invocations.

[markj]

Fix genassym invocations in sys/modules

[emaste]

I guess this is a good indication that we should find a common way to do this. But I think it's fine to commit this to fix the immediate issue and see if @imp or @sjg has a suggestion.

[emaste]

We could handle the kernel modules via bsd.kmod.mk, at least.