Looking for best method to build for arm-glibc-2.13

[ppenguin]

I want to target NAS hardware with a dotnet5 app, which goes as low as armhf on glibc-2.13 (and pretty much all of them are gcc-4.x), even on recent NAS.

Publishing for any arm.* out of the box seems to support systems which were compiled on gcc >= 5.1 (new C++11 ABI) and have glibc >= 2.16.

I have solved the gcc part of the problem by building libstdc++.so.6 in an arm-glibc-2.13 container and just bundling it with the app+runtime (and setting LD_LIBRARY_PATH).

This appears to work on systems with glibc >= 2.16 (? only tested down to 2.17), but in my glibc-2.13 container it shows:

Failed to load /build/source-build/.dotnet/shared/Microsoft.NETCore.App/5.0.13/libcoreclr.so, error: /lib/arm-linux-gnueabihf/libc.so.6: version `GLIBC_2.16' not found (required by /build/source-build/.dotnet/shared/Microsoft.NETCore.App/5.0.13/libcoreclr.so)

I would expect to solve this if I can build libcoreclr.so on a glibc-2.13 system (container), but obviously cannot bootstrap it as ./build.sh itself throws this error since it relies on libcoreclr.so.

Does that mean I need to cross-compile (on a system which has at least glibc-2.16), and if so, is it feasible to build the rootfs for the cross-compile with glibc-2.13? Would this work, or should I expect more blockers down the line?