Set __get_thread()->tid as part of clone().

This was previously done only in fork() and pthread_create(), but this left raw
clone() with an invalid cached tid. Since the tid is used for pthread routines,
this led to unstable behavior after clone().

Test: ltp clone01 (see bug for more)
Test: mmma bionic/tests
Test: bionic-unit-tests-static --gtest_filter=*fork*:*clone*

Bug: 32612735
Bug: 32305649
Change-Id: I30eae5a8024b4c5da65476fcadfe14c6db35bb79
diff --git a/libc/bionic/fork.cpp b/libc/bionic/fork.cpp
index ffe94f4..32ea255 100644
--- a/libc/bionic/fork.cpp
+++ b/libc/bionic/fork.cpp
@@ -36,9 +36,6 @@
 
   pthread_internal_t* self = __get_thread();
 
-  // Remember the parent pid and invalidate the cached value while we fork.
-  pid_t parent_pid = self->invalidate_cached_pid();
-
   int result = clone(nullptr,
                      nullptr,
                      (CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD),
@@ -47,10 +44,11 @@
                      nullptr,
                      &(self->tid));
   if (result == 0) {
+    // Update the cached pid, since clone() will not set it directly (as
+    // self->tid is updated by the kernel).
     self->set_cached_pid(gettid());
     __bionic_atfork_run_child();
   } else {
-    self->set_cached_pid(parent_pid);
     __bionic_atfork_run_parent();
   }
   return result;