LibC: Use our implementation of crti.o and crtn.o

We have had these for quite a while, but we didn't compile them, and
used GCC's version instead. Clang does not come with these, so we have
to provide our own implementation.

Our implementation follows what `musl` and `FreeBSD` do, so this should
work fine, even if documentation can hardly be found for them.

Userland/DynamicLoader/main.cpp

@@ -150,10 +150,4 @@ void _entry(int argc, char** argv, char** envp)
     ELF::DynamicLinker::linker_main(move(main_program_name), main_program_fd, is_secure, argc, argv, envp);
     VERIFY_NOT_REACHED();
 }
-
-void _fini();
-
-void _fini()
-{
-}
 }