Obscure bugfix affecting foreign import "wrapper" with non-word-sized
argument types (eg. Double) on Windows.  The list of arguments types
returned by dsFExport was the boxed types, rather than the unboxed
types, so dsFExportDynamic couldn't get the correct sizes of the
argument types to the stub function.

It's more correct now, but not totally correct (see the comment for
details).

Noticed by: Wolfgang Thaller.