First, system_libs are collected into a list. This is the same as
before.

Next, system_libs are filtered into external_system_libs, which is the
same list but without any libc, compiler_rt, etc.

At this point, if there are any external system libs, native library
directory paths are detected and added to lib_dirs.

Finally, extern_system_libs are filtered into resolved_system_libs,
which has full paths to all of the libraries. This is the list passed
into Compilation.

This makes the required changes noted by @ifreund in the code review for
this branch.