Port libc++

[mysterymath]

We need a reasonably complete, standards compliant C++ standard library with a license that allows statically linking against it without virally requiring inclusion of a license disclaimer. There are really only a handful of notable C++ library implementations, and only libc++ has a suitable license.

Unfortunately, llvm-libc is not yet complete enough to build libc++, so either upstream or downstream work would need to be done to fill out the SDK's libc such that libc++ could build. Ideally this would be done piecemeal.