app: add support for Linux backtrace() API.
It is nice because when available (Linux only?), it is a lot faster than using a dedicated debugger such as GDB or LLDB, and also it allows to always have a backtrace, even when no debuggers are installed. Unfortunately the output is a lot less detailed, with no file paths, no line numbers (even when debug symbols are there), no local values printout, etc. It's pretty bare, with function names and the stack levels. This is why it is not given priority, and GDB and LLDB are still preferred when available.
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