versioner: whitelist atexit, turn on symbol checking by default.

Change-Id: I32e726c74ee618ace3a4329d46408a42732a8d9d
diff --git a/tools/versioner/src/versioner.h b/tools/versioner/src/versioner.h
index 95635bb..ced9b79 100644
--- a/tools/versioner/src/versioner.h
+++ b/tools/versioner/src/versioner.h
@@ -20,6 +20,7 @@
 #include <set>
 #include <string>
 #include <unordered_map>
+#include <unordered_set>
 
 extern bool verbose;
 
@@ -55,3 +56,8 @@
   // time64.h #errors when included on LP64 archs.
   { "time64.h", { "arm64", "mips64", "x86_64" } },
 };
+
+static const std::unordered_set<std::string> missing_symbol_whitelist = {
+  // atexit comes from crtbegin.
+  "atexit",
+};