install: Correct libdir for Windows installs.

Platform-detection code from `configure` copied over to `install.sh`
in order to special case the lib dir being `bin` on Windows instead
of `lib`.

Short-term fix for #13810.
auto merge of #15087 : iliekturtles/rust/13810-make-install-mingw32, r=brson

Short-term fix per @brson's comment: https://github.com/rust-lang/rust/issues/13810#issuecomment-43562843. Tested on Win7 x64 and Linux.

One possible issue is that `install.sh` doesn't have a `need_cmd` definition like `configure` does. Should this be ported over as well?

Platform-detection code from `configure` copied over to `install.sh` in
order to special case the lib dir being `bin` on Windows instead of
`lib`.

Short-term fix for #13810.