add ipc pipe (#28)

kernel/syscall/syscall.c

@@ -779,7 +779,7 @@ system_call_t system_call_table[MAX_SYSTEM_CALL_NUM] =
         [17] = sys_mkdir,
         [18] = sys_nanosleep,
         [19] = sys_clock,
-        [20] = system_call_not_exists,
+        [20] = sys_pipe,
         [21] = sys_mstat,
         [22 ... 254] = system_call_not_exists,
         [255] = sys_ahci_end_req};