MFV r281278:

• Update xz to 5.2.1, where the most visible change is that it fixed a compression-ratio regression in fast mode LZMA1 and LZMA2 and used cpuset_getaffinity() for CPU cores detection.
• Make liblzma use the base system SHA256 implementation instead of the bundled one.
• Additional annotation in config.h for FreeBSD specific tweaks.
• Refresh symbols in XZprivate_1.0 to reflect reality.

Relnotes: yes
MFC after: 1 month (TBD)