MFV r357608: Limit memory usage in xz(1) instead of in tuklib.

Apply upstream 353970510895f6a80adfe60cf71b70a95adfa8bc to limit memory
usage on 32-bit binary to 4020 MiB.

Submitted by: Lasse Collin <lasse.collin at tukaani.org>
Reviewed by: kib, bcr
Differential Revision: https://reviews.freebsd.org/D23474