Add __pure2 to a few more functions, most notably gettid and pthread_self.

Change-Id: I7eee9f26f45130038af09d8285782b07f70a996f

benchmarks/pthread_benchmark.cpp

@@ -18,11 +18,14 @@
 
 #include <pthread.h>
 
+// Stop GCC optimizing out our pure function.
+/* Must not be static! */ pthread_t (*pthread_self_fp)() = pthread_self;
+
 static void BM_pthread_self(int iters) {
   StartBenchmarkTiming();
 
   for (int i = 0; i < iters; ++i) {
-    pthread_self();
+    pthread_self_fp();
   }
 
   StopBenchmarkTiming();