resolve_windows_libs: add llvm/mingw support

The existing code tries to mirror how the linker finds DLLs by
searching for the import libs and then looking for the matching
shared lib name.

While llvm has a dlltool clone it doesn't provide the --identify
option to extract the shared lib name. Instead we use the fact that
llvm import libs include the dll name in the archive member name, so
we can use "nm" there to get the same result.

To decide which strategy to use we run dlltool and check if it contains
"llvm-dlltool" in the output.

This fixes the .gir and .typelib files containing bogus values for the
shared library names when building with clang + mingw-w64 on Windows.

I'm not quite sure if the libtool part is actually needed there,
but I left it in to keep the diff small.