[libc][NFC] Use a end of list marker for cpu feature detection.

Without this, the array can end up being an empty array leading to
compiler failures.

libc/cmake/modules/cpu_features/check_cpu_features.cpp.in

@@ -18,6 +18,9 @@
 int main(int, char **) {
   const char *strings[] = {
       @DEFINITIONS@
+      // If DEFINITIONS turns out to be empty, we don't want to list
+      // an empty array. So, we add an end of list marker.
+      "<end_of_feature_list>"
   };
   const size_t size = sizeof(strings) / sizeof(strings[0]);
   for (size_t i = 0; i < size; ++i) {