aarch64: add struct _aarch64_ctx to signal.h

[musl] / arch / aarch64 / bits / sem.h

struct semid_ds {
        struct ipc_perm sem_perm;
        time_t sem_otime;
        time_t sem_ctime;
        time_t sem_nsems;
        time_t __unused3;
        time_t __unused4;
};