Import libcxxrt e64e93fe5bba67a6d52cbe5a97f8770c054bfa65

Implements __cxa_throw_bad_array_new_length, which is generated by gcc
4.9 and later. Also, some demangle support and fixes.

Import libcxxrt b2396b5945d7a2697c4762c3e52dc6f732b2eebd from
https://github.com/pathscale/libcxxrt/.