[mlir] mlir-cpu-runner test's cblas_interface should export functions on Windows

This change fixes the build on Windows, so that cblas_interface.dll
exports functions correctly and an implib is created and installed
correctly.

Currently, LLVM cannot be consumed on Windows after it has been
installed in a location because cblas_interface.lib is not
created/installed, thus failing the import check in `LLVMExports.cmake`.

Differential Revision: https://reviews.llvm.org/D72384