added instructions to utilize riscv64-unknown-elf-gdb

[U007D]

Following the directions did not run the leds_blink example using Rust v1.36.0/Pop!_OS; added instructions to specify the RISC-V toolchain's gdb to fix the issue.

[Disasm]

Thanks! However, I think that it's better to describe how to add it to PATH. If you change .cargo/config this change will make your project not portable (difficult to run on another machine) or you will have to avoid committing .cargo/config every time if you want to share your project.

[almindor]

@Disasm I think this should get merged, I just got back and this step actually got me fishing in memory too