Pass caller names to __pthread_internal_find for better errors.

On http://b/122082295 we had this abort:

  12-27 15:29:31.237 10222 10814 10848 F libc    : invalid pthread_t 0xb1907960 passed to libc

This wasn't super helpful. We can do better. Now you get something like
this instead:

  03-27 02:34:58.754 25329 25329 W libc    : invalid pthread_t (0) passed to pthread_join

Test: adb shell crasher
Bug: http://b/123255692
Change-Id: I1d545665a233308480cc3747ec3120e2b6de0453
diff --git a/libc/bionic/pthread_gettid_np.cpp b/libc/bionic/pthread_gettid_np.cpp
index 1beddc9..d14900b 100644
--- a/libc/bionic/pthread_gettid_np.cpp
+++ b/libc/bionic/pthread_gettid_np.cpp
@@ -31,6 +31,5 @@
 
 __BIONIC_WEAK_FOR_NATIVE_BRIDGE
 pid_t pthread_gettid_np(pthread_t t) {
-  pthread_internal_t* thread = __pthread_internal_find(t);
-  return thread ? thread->tid : -1;
+  return __pthread_internal_gettid(t, "pthread_gettid_np");
 }