Is there an open source program for Windows that offers the same functionality as Linux' /lib/ld‑linux.so.2?
The loader is a core part of the OS on Windows; there's no open-source alternative I'm aware of, and I'm not sure it'd be possible to do it correctly in any case - you have to handle the minefield of assumptions that kernel32/ntdll have regarding address space layout, support SxS, ASLR, hotpatching, and more.
Open-source linkers are common (e.g., gnu tools), but I gather that's not what you're after.