part of Issue#994

mcexec: open syscall moves to arch_dep
do_fork: don't use __NR_fork. use __NR_clone
vfork: moves to arch_dep

executer/user/arch/arm64/arch_syscall.c

@@ -0,0 +1,7 @@
+struct syscall_wait_desc;
+
+int
+archdep_syscall(struct syscall_wait_desc *w, long *ret)
+{
+	return -1;
+}