This changes add_custom_libcxx to also build libcxxabi and merges
the two into a static and hermetic library.
There are multiple advantages:
- The resulting libFuzzer doesn't expose C++ internals and looks like a plain C library.
- We don't have to manually link in libstdc++ to provide cxxabi.
- The sanitizer tests cannot interfere with an installed version of libc++.so in LD_LIBRARY_PATH.