Split creating host tools as a separate build target

By having host tools as a separate build target, we would end up only creating host tools 3 times, once for linux, windows and macos.

This would allow us to combine 9 Android builds into 3. We currently run Android cross compilations for 3 different architectures on mac, windows and linux. By having host tools available, we could bundle things so that host tools are quickly done on all 3 platforms, but the cross compilation is only done on the fastest platform once...I.e. linux. Then Qt installer could join up the host tools for the requested platform and the Android binaries making the end users environment complete.

What I don't know is if we really need to test that our host tools can be generated with all different GCC versions or if there's a good reason to do them with both VS2017 and VS2015? Or would it be enough that we did them once with just about anything. Meaning that in the end we did a VS2015 build with host tools generated with VS2017. Or in linux world host tools generated with GCC 9 would be used to run a GCC7 build.