Lock on dl_interate_phdr

There is possibility of someone dlclosing a library
while dl_iterate_phdr in progress which can lead to
dl_iterate_phdr calling callback with invalid address
if it was unmapped by dlclose.

Bug: http://b/22047255
Change-Id: I3fc0d9fd2c51fb36fd34cb035f37271fa893a7be
(cherry picked from commit f7d5bf334d)

linker/dlfcn.cpp

@@ -158,6 +158,11 @@ int dlclose(void* handle) {
   return 0;
 }
 
+int dl_iterate_phdr(int (*cb)(dl_phdr_info* info, size_t size, void* data), void* data) {
+  ScopedPthreadMutexLocker locker(&g_dl_mutex);
+  return do_dl_iterate_phdr(cb, data);
+}
+
 void android_set_application_target_sdk_version(uint32_t target) {
   // lock to avoid modification in the middle of dlopen.
   ScopedPthreadMutexLocker locker(&g_dl_mutex);