BuildSystem: Move all userpace libraries under the userspace directory

As the number of libraries is increasing, root directory starts to
expand. This adds better organization for libraries

userspace/libraries/LibC/include/icxxabi.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+int __cxa_atexit(void (*func)(void*), void* data, void* dso_handle);
+void __cxa_finalize(void* func);
+
+__END_DECLS