libcxxrt: Insert padding in __cxa_dependent_exception

Padding was added to cxa_exception in 45ca8b19 and
cxa_dependent_exception needs the same layout.
Add some static_asserts to detect this in the future.

Merge of libcxxrt commit b00c6c564357

(cherry picked from commit c40e4349889b32500e51e60f9529dbcc080f468b)

Approved by: re (gjb, accelerated MFC)