Add all compatible system types directory to module paths

fixes #12915
closes #12916

Since Spack has support for specific targets it might happen that
software is built for targets that are not exactly the host because
it was either an explicit user request or the compiler being used is
too old to support the host.

Modules for different targets are written into different directories
and by default Spack was adding to MODULEPATH only the directory
corresponding to the current host. This PR modifies this behavior to
add all the directories that are **compatible** with the current host.