Frankly, I do not understand who would think glibc symbols themselves would be the challenge in this case. Even if you statically link glibc there's zero guarantee the syscalls will be present in the older Linux (cue .ABI-tag failures). Or even damn ELF format changes (e.g. gnu-style hashes). The simple solution is to build in the older Linux (&glibc).

In my long experience with ancient binaries, glibc has almost never been the problem, and its ability to _run_ ancient binaries is all but excellent; even Linux is more of a problem than glibc is (for starters paths to everywhere in /proc, /sys change every other half-decade).