This allows libmono to load extra libraries (notably win-iconv) from system_dllpath.
For example, Fedora's wine-mono is built against a shared win-iconv installed in system_dllpath, but wine will not search there unless it's explicitly instructed to do so since bfbccf1a038e (ntdll: Prevent loading Wine system dependencies in place of identically named application DLLs.).