This is (an incomplete) support for building compiler-rt builtins,
libunwind, libc++abi and libc++. The library build should be complete,
but not all CMake options have been replicated in GN.
We always use the just built compiler to build all the runtimes, which
is equivalent to the CMake runtimes build. This simplifies the build
configuration because we don't need to support arbitrary host compiler
and can always assume the latest Clang. With GN's toolchain support,
this is significantly more efficient than the CMake runtimes build.