Merge libcxxrt master 03c83f5a57be8c5b1a29a68de5638744f17d28ba
Interesting fixes (* were already cherry-picked):
- 03c83f5 add __cxa_init_primary_exception (#23)
- 5d8a158 Fix two bugs in __cxa_end_cleanup()
- b00c6c5 Insert padding in __cxa_dependent_exception
- 45ca8b1 Insert padding in __cxa_exception struct for compatibility
- f2e5509 Fix unlock in two-word version and add missing comment.
- 6229590 Add an option for disabling emergency buffers. (#14)
MFC after: 2 weeks