Port liblzma to the kernel

[cem]

Summary

Integrate liblzma with the kernel build system. Note that this assumes the
contrib/xz directory will be relocated to sys/contrib first. The kernel
build system can only access files in <root>/sys, apparently.

Mock up headers and macros to get appropriate defines for liblzma; avoid
adding a duplicate crc32 implementation.

[cem]

Remove Isilon-internal annotations and dependencies (ASSERT())

[cem]

I would still like to commit this at some point. A GZIO-alike wrapper can be written (like the ZSTDIO I recently proposed), and I think it would be a useful configuration for userspace core dumps. I don't think lzma will be useful for compressing kernel dumps, because there is no way to allocate a static context, AFAICT.

[cem]

Might be interested for someone who really wants better compressed user cores, but zstd suffices for me.