Fix install directory on Posix systems

Currently, tools are installed into `$(INSTALL_DIR)/bin` but they should be installed into `$(INSTALL_DIR)/bin$(MODEL)` on Posix systems (except Mac OS X).

This pull request makes consistent with the corresponding requests for [#3798 for dmd](https://github.com/D-Programming-Language/dmd/pull/3798), [#908 for druntime](https://github.com/D-Programming-Language/druntime/pull/908), and [#2362 for phobos](https://github.com/D-Programming-Language/phobos/pull/2362).

It says `mkdir -p $(INSTALL_DIR)/$(OS)/$(bin_dir)` for dmd, are you missing the `$(OS)` here?