sparc64: "undefined symbol '__builtin_unreachable'" - any suggestions?

[Stuart Henderson]

I'm looking at a report of certbot (python letsencrypt client) failing
here:  https://github.com/certbot/certbot/issues/6019

The reporter is trying to use it on 6.3 sparc64 and running into this:

python2.7:/usr/local/lib/python2.7/site-packages/cryptography/hazmat/bindings/_openssl.so:
 undefined symbol '__builtin_unreachable'

To trigger:

# pkg_add py-cryptography
$ python2.7 -c 'from cryptography.hazmat.bindings._openssl import ffi, lib'

Can people with various non-x86 arches try that and let me know which
work/fail please?

Any ideas for a way to fix it? The code involved is for things
like below (some are more complex functions but all involved with
per-OS-selection for RNG), they look like they're probably all
patchable if needed, though that wouldn't be the cleanest workaround.

static const char *osurandom_get_implementation(void) {
    switch(getentropy_works) {
    case CRYPTOGRAPHY_OSRANDOM_GETENTROPY_FALLBACK:
        return "/dev/urandom";
    case CRYPTOGRAPHY_OSRANDOM_GETENTROPY_WORKS:
        return "getentropy";
    }
    __builtin_unreachable();
}