maint: use an optimal-for-coreutils xz compression setting

maint: use an optimal-for-coreutils xz compression setting

* cfg.mk: Set XZ_OPT = -8e (determined empirically).
This sacrifices 8 KiB of compressed tarball size for a 32-MiB
decrease in the memory required during decompression.  I.e.,
using -9e would shave off only 8 KiB from the tar.xz file, yet
would force every decompression process to use 32 MiB more memory.