Build: Install most headers to Root (and libcore.a/libgui.a)

This makes out-of-tree linking possible. And at the same time, add a
CMakeToolchain.txt file that can be used to build arbitrary cmake-using
applications on Serenity by pointing to the CMAKE_TOOLCHAIN_FILE when
running cmake:

    -DCMAKE_TOOLCHAIN_FILE=~/code/serenity/Toolchain/CMakeToolchain.txt

Kernel/sync.sh

@@ -6,7 +6,7 @@ fi
 
 if [ $(id -u) != 0 ]; then
     echo "This needs to be run as root"
-    exit
+    exit 1
 fi
 
 rm -vf _fs_contents.lock