crossdirect: fix 'ld: cannot find -lz'

Create a /usr/armv6-alpine-linux-muslgnueabihf/lib -> /lib symlink in buildroot_armhf as workaround for crossdirect bug pmaports#227 (closed).

How to test

Run pmbootstrap build --arch=armhf --force --strict coreinfo without and with this patch. With this patch it will build successfully.

Fixes pmaports#227 (closed).

CC: @zhuowei (FYI)