c-shared buildmode is currently only supported on Linux.
As seen here, Building a dll with Go 1.7, you may be able to build a DLL or shared lib using gcc, but it's not supported directly by the Go toolchain.
However, if you're looking to create a "plugin" architecture, you don't want to create a C shared library, in which each instance creates a new Go runtime. There is already a
plugin package which is intended for that purpose.