Log Message:
lang: Import libcxxabi version 6.0.1.

libc++abi is a new implementation of low level support for a standard
C++ library.

Features and Goals
 * Correctness as defined by the C++11 standard.
 * Provide a portable sublayer to ease the porting of libc++
 * On Mac OS X, be ABI compatible with the existing low-level support.