Fix macro Crystal::LIBRARY_PATH.split when cross-compiling

[HertzDevil]

Crystal::LIBRARY_PATH always uses the host platform's path delimiter, but Process::PATH_DELIMITER is defined in terms of the target platform, which can break cross-compilation from a non-Windows system to Windows when CRYSTAL_LIBRARY_PATH has multiple items. This PR adds the correct delimiter as a constant.

By the way, this is actually the first usage of host_flag? in the repository.

[straight-shoota]

I'm wondering if Process is the best namespace for HOST_PATH_DELIMITER. Of course, there's some affinity to Process::PATH_DELIMITER. But it's used quite differently. As far as I can see, it's only meant as a compile-time constant and typically would not be used at runtime. It's also only for internal use.
So perhaps Crystal (or Crystal::System?) might be a better namespace for this? Would fit right next to Crystal::HOST_TRIPLE.