clang: Build with gcc 4.7 for better libstdc++.
all-packages: Add a self-hosted variant of clang.
clang: Do not try to build clang-tblgen in LLVM's build path.
libcxx: Update to 190100 and build with libc++abi.
clang-wrapper: Guard against LD_LIBRARY_PATH.
clang-wrapper: Optionally use libc++ instead of libstdc++.