crossdirect: .../ld: cannot find -lz

Looks like we're not through with fixing all crossdirect bugs just yet 😉

Compiling the current coreapps MR for armhf fails at the "coreinfo" package with:

...
moc coreinfo.h                                                                                                                                   $
moc eventfilter.h
compiling build/moc-qt5/moc_eventfilter.cpp
compiling build/moc-qt5/moc_coreinfo.cpp
linking coreinfo
/native/usr/bin/../lib/gcc/armv6-alpine-linux-muslgnueabihf/8.3.0/../../../../armv6-alpine-linux-muslgnueabihf/bin/ld: cannot find -lz

I'll use --no-crossdirect in the binary repository for now, so merging that MR (and others like the current KDE upgrade, which might run into the same issue) is not blocked.

But fixing this is a high priority of mine. (When I'm confident that crossdirect is working properly, I'll tag a new pmbootstrap version and I want to do that soon.)

CC @zhuowei: FYI