Let memcmp correspond to the C standard (fix #628)

By now, the memcmp implementation of Genode's basic string utilities just
returned whether two memory blocks are equal or differ. It gave no hint which
block is greater, or lesser than the other one. This isn't the behaviour
anticipated by implementations that rely on the C standard memcmp, e.g. GCC's
libsupc++, or the nic_bridge's AVL tree implementation.