You are right, including "gcry_crc" in addition to "xzio" (and its dependent
"crypto"), and compressing with "xz -C crc32" (as mentioned in the manual [1])
works similarly to including "gzio" and compressing with "gzip -9".
I still get an "ELF magic" error for some modules, probably the same as with
gzio -- will try to figure out which ones these are exactly.