be explicit about the GDB script to use

with this change the user doesn't need to set an [auto-load-safe-path]

[auto-load-safe-path]: https://sourceware.org/gdb/onlinedocs/gdb/Auto_002dloading-safe-path.html

.cargo/config

@@ -1,5 +1,5 @@
 [target.'cfg(all(target_arch = "arm", target_os = "none"))']
-runner = 'arm-none-eabi-gdb'
+runner = 'arm-none-eabi-gdb -x debug.gdb'
 rustflags = [
   # LLD (shipped with the Rust toolchain) is used as the default linker
   "-C", "link-arg=-Tlink.x",