_MINIX_SYSTEM - fixes for in-minix crossbuild

. define _MINIX_SYSTEM for all system code from minix.service.mk
	. hide some system-level declarations and definitions
	  behind _MINIX_SYSTEM to cleanly fix host tool build problems on
	  Minix (such as: NONE being defined and paddr_t being used but not
	  declared)
	. the similar definition _SYSTEM is unsuitable as it changes the
	  values of errno definitions

Change-Id: I407de79e2575115243a074b16e79546a279cfa3e

lib/libc/sys-minix/stack_utils.c

@@ -3,6 +3,8 @@
  * Author: Lionel A. Sambuc. 
  */
 
+#define _MINIX_SYSTEM
+
 #include <sys/cdefs.h>
 #include "namespace.h"
 #include <lib.h>