When cross-compiling, flags which are passed to the compiler-rt build should be
passed to the custom libc++ build too.
What about C++ flags? libc++ is a C++ library so those are likely going to be important as well. Rather than lumping both C and C++ flags together into a single variable, we should probably consider adding two new keyword arguments to add_custom_libcxx, CMAKE_C_FLAGS and CMAKE_CXX_FLAGS, and use those to pass C and C++ flags separately to libc++ build.