Allow more customisation of gdb for replay

[huonw]

It would be nice if one could customize the gdb program that rr replay runs, or pass extra arguments through to it. For instance, it is much nicer to debug Rust via the rust-gdb wrapper, rather than raw gdb itself, since it sets up loading some Python pretty printer scripts. I imagine it could be something like GDB=custom rr replay ... or rr replay --gdb custom ... or something like that.