Add basic stacktrace handler using libunwind.

This means we will all have to apt-get install libunwind8-dev on Linux.  Mac comes with everything we need already.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/343583005
diff --git a/bench/benchmain.cpp b/bench/benchmain.cpp
index 6e18a8a..078a2e8 100644
--- a/bench/benchmain.cpp
+++ b/bench/benchmain.cpp
@@ -6,6 +6,7 @@
  */
 
 #include "BenchTimer.h"
+#include "CrashHandler.h"
 #include "ResultsWriter.h"
 #include "SkBenchLogger.h"
 #include "SkBenchmark.h"
@@ -271,6 +272,7 @@
 
 int tool_main(int argc, char** argv);
 int tool_main(int argc, char** argv) {
+    SetupCrashHandler();
     SkCommandLineFlags::Parse(argc, argv);
 #if SK_ENABLE_INST_COUNT
     if (FLAGS_leaks) {