syscall_plain
void syscall_plain(struct lwp *, u_int64_t, struct trapframe *);
p->p_md.md_syscall = syscall_plain;
p->p_md.md_syscall = syscall_plain;
p->p_md.md_syscall = syscall_plain;
static void syscall_plain(register_t, struct lwp *, struct frame *);