don't copy win64 struct args

doc/libffi.texi

@@ -171,7 +171,9 @@ discarded.
 @var{avalues} is a vector of @code{void *} pointers that point to the
 memory locations holding the argument values for a call.  If @var{cif}
 declares that the function has no arguments (i.e., @var{nargs} was 0),
-then @var{avalues} is ignored.
+then @var{avalues} is ignored.  Note that argument values may be
+modified by the callee (for instance, structs passed by value); the
+burden of copying pass-by-value arguments is placed on the caller.
 @end defun