versioner: Update libc.map.txt to match annotations

This commit removes several symbol versions (API 14 and 15) from
`libc.map.txt` because we no longer support NDK with those API levels.
This also matches the versioner annotations in the header files.

This commit also annotates twalk() with __INTRODUCED_IN(21).  It was
accidentally removed in aosp/1157510.

Test: source development/vndk/tools/header-checker/android/envsetup.sh && \
      source build/envsetup.sh && \
      lunch aosp_arm64-userdebug && \
      m versioner && \
      ./bionic/tools/versioner/run_tests.py
Change-Id: I211fe5b7b1b66793d5e76a8676f9d18825f96b5e
diff --git a/libc/include/search.h b/libc/include/search.h
index 7a75404..7c4989a 100644
--- a/libc/include/search.h
+++ b/libc/include/search.h
@@ -214,6 +214,6 @@
  * [twalk(3)](http://man7.org/linux/man-pages/man3/twalk.3.html) calls
  * `__visitor` on every node in the tree.
  */
-void twalk(const void* __root, void (*__visitor)(const void*, VISIT, int));
+void twalk(const void* __root, void (*__visitor)(const void*, VISIT, int)) __INTRODUCED_IN(21);
 
 __END_DECLS