This reverts commit 8db56defa766eacdbaf89a37f25b11a57fd9787a, reversing
the xz 5.6.0 vendor import.
Another commit will follow to restore a symbol introduced in 5.6.0 to
preserve binary interface.
Backport export of lzma_mt_block_size symbol.
This restores binary compatibility against xz 5.6.0 library.