cross/crossdirect: use qemu when linking
Cross linker (e.g. ld from binutils-armhf) does not support any method of setting additional link paths, and this causes problems when building various packages. Work around this problem by calling the qemu binary instead of the cross binary from the native chroot, whenever we are linking.
I have tested that this allows successfully building hello-world, coreinfo, libhybris for armhf. So this fixes #227 (closed) and fixes #228 (closed).
Regarding performance, this is the same way as distcc does it. We would get a performance gain if we were able to use the cross-linker. But so far I could not figure out how to patch the binutils source to make it work like we want to have it. Maybe I'll ask on some binutils mailing list for advice. In the meantime, this workaround should make crossdirect work for all use-cases, and it is still faster than the previous distcc setup, because we can avoid the overhead of setting up and running a ssh server locally to work around missing authentication in distcc (see pmbootstrap!1649 (merged)). Let's test this for some more time, and then make a pmbootstrap release that rolls out crossdirect for everyone.
As I have tested this extensively, and it's an important fix, I'm merging this as soon as CI goes through.
CC: @zhuowei: FYI