Installing from opkg gets me libstdcpp6_5.5.0-3_arm_cortex-a7.ipk . However, this library is too old and results in runtime errors in zerotier:

Error relocating /usr/bin/zerotier-idtool: _ZNSt6thread15_M_start_threadESt10unique_ptrINS_6_StateESt14default_deleteIS1_EEPFvvE: symbol not found
Error relocating /usr/bin/zerotier-idtool: _ZNSt6thread6_StateD2Ev: symbol not found
Error relocating /usr/bin/zerotier-idtool: _ZTINSt6thread6_StateE: symbol not found