Introduce experimental debug support for Arduino CLI/IDE

In the SAMD platform definition we have introduced experimental support to allow debug using the new IDE.
@cmaglie has worked on it, but the bulk of it is adding this in the `platform.txt` with the correct parameters

```# Debugger configuration (general options)
# ----------------------------------------
# EXPERIMENTAL feature:
# - this is alpha and may be subject to change without notice
debug.executable={build.path}/{build.project_name}.elf
debug.toolchain=gcc
debug.toolchain.path={runtime.tools.arm-none-eabi-gcc-7-2017q4.path}/bin/
debug.toolchain.prefix=arm-none-eabi-
debug.server=openocd
debug.server.openocd.path={runtime.tools.openocd-0.10.0-arduino7.path}/bin/openocd
debug.server.openocd.scripts_dir={runtime.tools.openocd-0.10.0-arduino7.path}/share/openocd/scripts/
debug.server.openocd.script={runtime.platform.path}/variants/{build.variant}/{build.openocdscript}
```

This request comes from an issue filed in the Arduino Pro IDE public repo
https://github.com/arduino/arduino-pro-ide/issues/301

Transferred here as requested by @per1234

Start with the SAMD platform definition and its platform.txt file, then compare the requested debugger configuration with the existing platform parameters. Confirm the paths and placeholders for the ELF executable, ARM toolchain, OpenOCD, and variant script, and consider the Arduino Pro IDE issue for context. Done means the experimental debug settings are present in platform.txt.

Written by the indexing model from the issue text.