Append target-specific tools directory ($(RUST)/bin/rustlib/<triple>/bin/) to PATH during linking,
so that rustc can invoke them.
Update license notice.
Package rustc's mingw dependencies into Windows installer.
gcc, ld, ar, dlltool, windres go into $(RUST)/bin/rustlib/<triple>/bin/
platform libraries and startup objects got into $(RUST)/bin/rustlib/<triple>/lib/