Link with prebuilt OpenBLAS and fix shared libs build on Windows

[zcbenz]

Close #3031.
Close #3020.

This PR links MLX with the prebuilt dynamic library of OpenBLAS on Windows instead of building it from source code, which can save a lot of build time. (Note that we can not do static-linking, check the comments for more.)

Which then requires changing the locations where libraries and executables are generated, as Windows requires the DLL and EXE files be in the same directory (as there is no equivalent of RPATH on Windows):

• For building: the DLL and EXE would be written in the root build dir (previously EXE are put in sub-dirs like tests/test.exe)
• For installing: the DLL would be put in the same dir with extension file (previously they are put in the bin sub-dir)

Note that the locations are only changed for Windows, and only when MLX is the top project, for projects that include MLX as a third party library there is nothing changed.

/cc @dhiltgen

[awni]

Looks great!