iOS: Disable os.execute() when building for iOS >= 8.0.

Thanks to Vyacheslav Egorov.

src/lib_os.c

@@ -36,7 +36,7 @@
 
 LJLIB_CF(os_execute)
 {
-#if LJ_TARGET_CONSOLE
+#if LJ_NO_SYSTEM
 #if LJ_52
   errno = ENOSYS;
   return luaL_fileresult(L, 0, NULL);

src/lj_arch.h

@@ -426,6 +426,9 @@
 #if defined(__symbian__)
 #define LUAJIT_NO_EXP2
 #endif
+#if LJ_TARGET_CONSOLE || (LJ_TARGET_IOS && __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_8_0)
+#define LJ_NO_SYSTEM		1
+#endif
 
 #if defined(LUAJIT_NO_UNWIND) || defined(__symbian__) || LJ_TARGET_IOS || LJ_TARGET_PS3 || LJ_TARGET_PS4
 #define LJ_NO_UNWIND		1