That's not much help when the function being called is in a shared system library like ld.so or libc that needs to support being used both by older binaries tha

That's not much help when the function being called is in a shared system library like ld.so or libc that needs to support being used both by older binaries that use SSE and newer ones that use AVX, which I think is the case here. People aren't just going to recompile every single program on the planet and break backwards compatibility with older systems in the process, so on some level I think there's no alternative but to work around this. (Also, remember that SSE is used for even non-vectorized floating point in modern software, and that a lot of the vectorized stuff is likely to be hand-optimized assembler.)