Have you included it as a dependency in your Makefile for the package?
As far as I know, not only is libstdc++ not installed by default, but the libc that is typically used (musl) is very compact and may be missing certain calls and features compared to libc intended for machines with "plenty of disk space"
This happens to me whenever I build c++ projects in openwrt with external toolchain and glibc (not sure which is the root cause).
Is your libstdcpp.provides file empty ? This is how I solve it: