ART: Use thread_local on the host for ART Thread*

Use thread_local instead of a pthread key for Thread::Current().
Retain the pthread key as we are still using it for some sanity
checks in ThreadExitCallback.

Bug: 138329277
Test: m test-art-host
Change-Id: Idfe43bc279459e307e2165d3a3762af09230bdfd
diff --git a/runtime/thread_list.cc b/runtime/thread_list.cc
index c971183..2fe239a 100644
--- a/runtime/thread_list.cc
+++ b/runtime/thread_list.cc
@@ -1485,6 +1485,7 @@
   __get_tls()[TLS_SLOT_ART_THREAD_SELF] = nullptr;
 #else
   CHECK_PTHREAD_CALL(pthread_setspecific, (Thread::pthread_key_self_, nullptr), "detach self");
+  Thread::self_tls_ = nullptr;
 #endif
 
   // Signal that a thread just detached.