Add stub implementations of unused syscalls explicitly (#1966)

source/Core/Src/syscalls.c

@@ -11,4 +11,14 @@
 /* Functions */
 void initialise_monitor_handles() {}
 
+/* Syscalls (stub implementations to avoid compile warnings and possibe future problems) */
 int _getpid(void) { return 1; }
+
+#if defined(MODEL_Pinecil) || defined(MODEL_Pinecilv2)
+// do nothing here because some stubs and real implementations added for Pinecils already
+#else
+off_t   _lseek(int fd, off_t ptr, int dir) { return -1; }
+ssize_t _read(int fd, void *ptr, size_t len) { return -1; }
+ssize_t _write(int fd, const void *ptr, size_t len) { return -1; }
+int     _close(int fd) { return -1; }
+#endif