We'll use this to prevent repeating common tool dependencies. They all
depend on LibCore and AK only. We also want to encapsulate common
install rules for them.
This allows us to remove all the add_subdirectory calls from the top
level CMakeLists.txt that referred to targets linking LagomCore.
Segregating the host tools and Serenity targets helps us get to a place
where the main Serenity build can simply use a CMake toolchain file
rather than swapping all the compiler/sysroot variables after building
host libraries and tools.
