Stack backtrace on MEGA65 using MOS-LLVM

For the MEGAphone, I wanted to make my software debugging on the MEGA65 easier. Stack back-traces are a great way to help debug errors, but we don't have gdb or lldb or anything like that on the MEGA65. m65dbg and related tools can help here, but they don't have an easy way to provide the complete call stack. To solve this for my needs, I added function instrumentation using the following in my…