eglibc: libstdc++ is not part of the C library

For some reason, the eglibc.mk file was trying to install libstdc++.so
from eglibc to the target. But the C++ standard library is provided by
GCC, not by the C library.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>