MFC r315947:

Import libcxxrt master 8a853717e61d5d55cbdf74d9d0a7545da5d5ff92.

Interesting fixes which were not already merged:
0c7c611 Merge C++ demangler bug fixes from ELF Tool Chain (#40)
2b208d9 __cxa_demangle_gnu3: demangle 'z' as '...', not 'ellipsis' (#41)