More libbacktrace fixes.

Included in minor fix ups is the addition of a warning macro to replace
all of the ALOGW calls.

Fix a race where multiple threads could be attempting to unwind the threads
of the current process at the same time.

Bug: 8410085

Change-Id: I02a65dc778dde690e5f95fc8ff069a32d0832fd1
diff --git a/libbacktrace/Backtrace.h b/libbacktrace/Backtrace.h
index b89bc89..00f0a10 100644
--- a/libbacktrace/Backtrace.h
+++ b/libbacktrace/Backtrace.h
@@ -21,6 +21,10 @@
 
 #include <sys/types.h>
 
+// Macro to log the function name along with the warning message.
+#define BACK_LOGW(format, ...) \
+  ALOGW("%s: " format, __PRETTY_FUNCTION__, ##__VA_ARGS__)
+
 class BacktraceImpl {
 public:
   virtual ~BacktraceImpl() { }