Previously, musl would reject the call, because the main stack has

ss_size == 0 and ss_flags == SS_DISABLE.

We could condition on ss_flags not containing SS_DISABLE, but instead,
simply remove the ss_size check, as the kernel performs the same check,
anyhow.

[[Note: I've also included the corresponding patch to libc-tests. I'm not sure if this is OK, or if I should send it separately?]]