Fix longdblsize and nvsize for 64-bit USE_LONG_DOUBLE builds on Windows

This fixes many test failures in op/pack.t, although two (13177 and 13180)
still remain.  The same two also fail with 32-bit USE_LONG_DOUBLE builds.

Thanks to sisyphus1@optusnet.com.au for spotting the failures and the
likely cause of them.