Makefile: remove unnecessary -lstdc++

Since our kernel includes the entirety of libstdc++, there's no point of
specifying "-lstdc++" while linking shared objects (like tests and cpio.so):
This "linking" doesn't actually do any linking, it just specifies that the
libstdc++ library is needed, but we already have it, needed or not.

The list of objects-needing-libstdc++ was not only unnecessary, but probably
also out of date, since we stopped bothering to update it after libstdc++
was added into the kernel.

So this patch removes these -lstdc++ lines.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>