Add better free tracking.

Included in this change:

- Change the tag when a pointer is freed so it's easy to detect if
  an already freed pointer is being used.
- Move the free backtrace out of the header. This backtrace is only
  used under only some circumstances, so no need to allocate space
  in all headers for it.
- Add new option free_track_backtrace_num_frames to specify how many
  frames to record when the free occurs. This removes the dependency
  on the backtrace option to get backtraces.

Bug: 26739265
Change-Id: I76f5209507dcf46af67ada162a7cb2bf282116f2
diff --git a/libc/malloc_debug/Config.h b/libc/malloc_debug/Config.h
index 4b91e2b..d2cc56d 100644
--- a/libc/malloc_debug/Config.h
+++ b/libc/malloc_debug/Config.h
@@ -61,6 +61,7 @@
   size_t expand_alloc_bytes = 0;
 
   size_t free_track_allocations = 0;
+  size_t free_track_backtrace_num_frames = 0;
 
   uint64_t options = 0;
   uint8_t fill_alloc_value;