external: update backward-cpp to v1.4

rename the directory name to backward-cpp

Closes: #803
Signed-off-by: Shengjing Zhu <i@zhsj.me>

external/backtrace-cpp/backward.cpp

@@ -1,32 +0,0 @@
-// Pick your poison.
-//
-// On GNU/Linux, you have few choices to get the most out of your stack trace.
-//
-// By default you get:
-//	- object filename
-//	- function name
-//
-// In order to add:
-//	- source filename
-//	- line and column numbers
-//	- source code snippet (assuming the file is accessible)
-
-// Install one of the following library then uncomment one of the macro (or
-// better, add the detection of the lib and the macro definition in your build
-// system)
-
-// - apt-get install libdw-dev ...
-// - g++/clang++ -ldw ...
-// #define BACKWARD_HAS_DW 1
-
-// - apt-get install binutils-dev ...
-// - g++/clang++ -lbfd ...
-// #define BACKWARD_HAS_BFD 1
-
-#include "backward.hpp"
-
-namespace backward {
-
-backward::SignalHandling sh;
-
-} // namespace backward