Run thread backtrace before loading Python modules in hang_analyzer.py on ARM platform

GDB on ARM platform can run out of virtual memory, when executing a backtrace, after the Python modules are loaded. As a work around, we will run the thread backtrace, thread apply all bt before loading the Python modules, on the ARM 64 platform (Ubuntu 16.04).

Note the platform can be identified as aarch64-unknown-linux-gnu