pthread_exit(): reduce duplication.

Strictly, this is more of "swap one form of duplication for another", but
I found the existing code non-obvious in part because people have added new
code under existing comments (which don't apply), in two places. At this
point, duplicating the _condition_ (which is much less likely to change at
all, let alone grow more complex) clarifies the code and makes the comments
match the code they're adjacent to again.

Test: treehugger
Change-Id: Ic8f01dc5b4fd14e942bf8dd7c72cab7df06d99d5
diff --git a/libc/bionic/pthread_exit.cpp b/libc/bionic/pthread_exit.cpp
index f584b27..0181aba 100644
--- a/libc/bionic/pthread_exit.cpp
+++ b/libc/bionic/pthread_exit.cpp
@@ -133,20 +133,17 @@
 
     // pthread_internal_t is freed below with stack, not here.
     __pthread_internal_remove(thread);
-
-    if (thread->mmap_size != 0) {
-      // We need to free mapped space for detached threads when they exit.
-      // That's not something we can do in C.
-      __notify_thread_exit_callbacks();
-      __hwasan_thread_exit();
-      _exit_with_stack_teardown(thread->mmap_base, thread->mmap_size);
-    }
   }
 
-  // No need to free mapped space. Either there was no space mapped, or it is left for
-  // the pthread_join caller to clean up.
   __notify_thread_exit_callbacks();
   __hwasan_thread_exit();
 
+  if (old_state == THREAD_DETACHED && thread->mmap_size != 0) {
+    // We need to free mapped space for detached threads when they exit.
+    // That's not something we can do in C.
+    _exit_with_stack_teardown(thread->mmap_base, thread->mmap_size);
+  }
+  // No need to free mapped space. Either there was no space mapped,
+  // or it is left for the pthread_join caller to clean up.
   __exit(0);
 }