IBM Z: Add vectorized CRC32 implementation

IBM Z: Add vectorized CRC32 implementation

While DFLTCC takes care of accelerating compression on level 1, other
levels can be sped up too by computing CRC32 using various vector
instructions.

Take the Linux kernel assembly code that does that - its original
author (Hendrik Brueckner) works for IBM at the time of writing and has
allowed reusing the code under the zlib license. Rewrite it in C for
better maintainability, but keep the original structure, variable names
and comments.

Update the documentation.

Add CI configurations.