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.
I'm not certain cblas_interface is the right name here, it doesn't seem to contain anything specific to BLAS. Maybe linalg_interface would have been a better name. May be a separate clean up commit after syncing with @nicolasvasilache