Remove libbacktrace definition of gettid.

Also simplifies the Darwin implementation of gettid, because
apparently libbacktrace had a better way of doing it.

Change-Id: I9e14f9176b167405a5aaa5b8a0db0327c7126271
diff --git a/libcutils/threads.c b/libcutils/threads.c
index 3d8dd48..036f8c5 100644
--- a/libcutils/threads.c
+++ b/libcutils/threads.c
@@ -35,12 +35,7 @@
 #ifndef __ANDROID__
 pid_t gettid() {
 #if defined(__APPLE__)
-  uint64_t owner;
-  int rc = pthread_threadid_np(NULL, &owner);
-  if (rc != 0) {
-    abort();
-  }
-  return owner;
+  return syscall(SYS_thread_selfid);
 #elif defined(__linux__)
   return syscall(__NR_gettid);
 #elif defined(_WIN32)