Windows cross-compilers should automatically be detected if hostOS does not contain "windows" and targetOS does

There are several well-known compiler triples for GCC-based Windows cross compilers, which can easily be installed via a Linux package manager. On non-Windows platforms, if the targetOS contains windows, the compiler probe should look for these triples in their well-known installed locations and use them if found.

This allows Windows projects to be built on Linux with a simple qbs.targetPlatform binding (and therefore with profile:none). Consequently, this brings us closer to being able to remove setup-toolchains entirely.