Disable Linux vDSO support for now

It's been encountering some strange errors that are difficult to
reproduce.

libc/calls/clock_gettime.c

@@ -80,6 +80,8 @@ noinstrument int clock_gettime(int clockid, struct timespec *ts) {
  * Returns fast system clock_gettime() if it exists.
  */
 void *__get_clock_gettime(void) {
+  // TODO(jart): Re-enable this.
+  return 0;
   void *vdso;
   static bool once;
   static void *result;