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/Corkscrew.cpp b/libbacktrace/Corkscrew.cpp
index 8ba1e80..9daa752 100644
--- a/libbacktrace/Corkscrew.cpp
+++ b/libbacktrace/Corkscrew.cpp
@@ -37,7 +37,7 @@
 bool CorkscrewCommon::GenerateFrameData(
     backtrace_frame_t* cork_frames, ssize_t num_frames) {
   if (num_frames < 0) {
-    ALOGW("CorkscrewCommon::GenerateFrameData: libcorkscrew unwind failed.\n");
+    BACK_LOGW("libcorkscrew unwind failed.");
     return false;
   }
 
@@ -184,7 +184,7 @@
 }
 
 //-------------------------------------------------------------------------
-// C++ object createion functions.
+// C++ object creation functions.
 //-------------------------------------------------------------------------
 Backtrace* CreateCurrentObj() {
   return new BacktraceCurrent(new CorkscrewCurrent());