Linux of course does have an interface for RISC-V extension probing via hwprobe. And there's a C interface[1] for probing that's OS-agnostic (though it's rather new).

[1]: https://github.com/riscv-non-isa/riscv-c-api-doc/blob/main/s...

It's not strictly x86 either, the other case you care about is fp16 support on ARM. But it is included in the M1 target, so really only on other ARM.