Avoid a system call in 'gettid'.

System calls can be pretty slow. This is mako, which has one of our
lowest latencies:

                       iterations      ns/op
  BM_unistd_getpid       10000000        209
  BM_unistd_gettid      200000000          8

Bug: 15297299 (kernel panic from too many gettid calls)
Bug: 15315766 (excessive gettid overhead in liblogd)
Change-Id: I49656c0fc5b5d092390264a59e4f2c0d8a8b1aeb

benchmarks/unistd_benchmark.cpp

@@ -28,3 +28,14 @@ static void BM_unistd_getpid(int iters) {
   StopBenchmarkTiming();
 }
 BENCHMARK(BM_unistd_getpid);
+
+static void BM_unistd_gettid(int iters) {
+  StartBenchmarkTiming();
+
+  for (int i = 0; i < iters; ++i) {
+    gettid();
+  }
+
+  StopBenchmarkTiming();
+}
+BENCHMARK(BM_unistd_gettid);