Add pthread_gettid_np and re-expose __get_thread for LP32.

A lot of third-party code calls the private __get_thread symbol,
often as part of a backport of bionic's pthread_rwlock implementation.
Hopefully this will go away for LP64 (since you're guaranteed the
real implementation there), but there are still APIs that take a tid
and no way to convert between a pthread_t and a tid. pthread_gettid_np
is a public API for that. To aid the transition, make __get_thread
available again for LP32.

(cherry-pick of 27efc48814b8153c55cbcd0af5d9add824816e69.)

Bug: 14079438
Change-Id: I43fabc7f1918250d31d4665ffa4ca352d0dbeac1
diff --git a/libc/bionic/pthread_gettid_np.cpp b/libc/bionic/pthread_gettid_np.cpp
new file mode 100644
index 0000000..f4663a7
--- /dev/null
+++ b/libc/bionic/pthread_gettid_np.cpp
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include "pthread_internal.h"
+#include "private/bionic_pthread.h"
+
+pid_t pthread_gettid_np(pthread_t t) {
+  return reinterpret_cast<pthread_internal_t*>(t)->tid;
+}
+
+// TODO: move callers over to pthread_gettid_np and remove this.
+pid_t __pthread_gettid(pthread_t t) {
+  return pthread_gettid_np(t);
+}