MinGW seems to decide at random whether it wants to use memcpy from
mscvrt.dll or ntdll.dll. Currently, on Debian buster, ntdll.dll is chosen,
while on sid, mscvrt.dll is chosen.
This commit declares a new .def file for ntdll containing only the
functions we want to link from ntdll.dll, and generates ntdll.a from it
with dlltool. This way, MinGW will never be tempted to link functions
like memcpy from ntdll.dll.
