xz: upgrade 5.4.6 -> 5.6.2

xz: upgrade 5.4.6 -> 5.6.2

This is the first post-backdoor release.

These are the release notes:
https://github.com/tukaani-project/xz/releases/

There are also backdoor notes:
https://tukaani.org/xz-backdoor/
"I plan to write an article how the backdoor got into the releases and
what can be learned from this." - that'd be most welcome, as it would
be first hand information that sets the record straight.

And there's a commit by commit review of Jia Tan's contributions:
https://tukaani.org/xz-backdoor/review.html

Add an option for landlock sandbox (off by default as it clashes with running under pseudo).

License-Update: public domain bits were relicensed under 0BSD license

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>