GDB debugging support

[jawilk]

Hi, I added basic gdb debugging support for the solana rbpf vm and this poc-framework here https://github.com/jawilk/solana-poc-framework and put a simple example here https://github.com/jawilk/solana-poc-debugging-example
Since I had to fork a few dependencies I think a pull request is not feasible, so just putting it here.
It might be useful for some if the projects unoptimized binary size isn't too big

[riptl]

@jawilk I think this is quite useful. Let's get this upstreamed or maintain a fork with the feature, wdyt?