We already try to annotate functions exported from DSOs as such.
However so far we were testing only on Linux and macOS where
-fvisibility=hidden is not the default, and thus linker forgives us
annotation mistakes. But on Windows the default is to have hidden
visibility out of the box and so the linker becomes strict about this.

-> Fix all issues caught there.

For example here is how it breaks if ~_interface() is not annotated properly:

    building 'golang.runtime.libpyxruntime' DSO as build\lib.win-amd64-cpython-310\golang\runtime\libpyxruntime.dll
    ...
    Z:\home\kirr\src\tools\go\pygo-win\BuildTools\vc\tools\msvc\14.35.32215\bin\Hostx64\x64\link.exe /nologo /INCREMENTAL:NO /LTCG /DLL /MANIFEST:EMBED,ID=2 /MANIFESTUAC:NO "/LIBPATH:C:\Program Files\Python310\li
    bs" /LIBPATH:build\lib.win-amd64-cpython-310\golang\runtime /LIBPATH:z:\home\kirr\src\tools\go\pygo-win\BuildTools\vc\tools\msvc\14.35.32215\lib\x64 /LIBPATH:z:\home\kirr\src\tools\go\pygo-win\BuildTools\kits
    \10\lib\10.0.22000.0\ucrt\x64 /LIBPATH:z:\home\kirr\src\tools\go\pygo-win\BuildTools\kits\10\lib\10.0.22000.0\um\x64 libgolang.lib build\temp.win-amd64-cpython-310\Release\golang/runtime/libpyxruntime.obj /OU
    T:build\lib.win-amd64-cpython-310\golang\runtime\libpyxruntime.dll /IMPLIB:build\lib.win-amd64-cpython-310\golang\runtime\libpyxruntime.lib
       Creating library build\lib.win-amd64-cpython-310\golang\runtime\libpyxruntime.lib and object build\lib.win-amd64-cpython-310\golang\runtime\libpyxruntime.exp

    libpyxruntime.obj : error LNK2001: unresolved external symbol "protected: __cdecl golang::_interface::~_interface(void)" (??1_interface@golang@@IEAA@XZ)
    build\lib.win-amd64-cpython-310\golang\runtime\libpyxruntime.dll : fatal error LNK1120: 1 unresolved externals
    error: command 'Z:\\home\\kirr\\src\\tools\\go\\pygo-win\\BuildTools\\vc\\tools\\msvc\\14.35.32215\\bin\\Hostx64\\x64\\link.exe' failed with exit code 1120