Feature: GD32VF1 support

[dragonmux]

Detailed description

This PR implements support for the GD32VF1 series processors, which is also our first RISC-V target. The GD32VF103 is the device against which the initial RISC-V support PR was tested.

This provides complete support for the Flash, memory maps, full debug, and even the option bytes.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (make PROBE_HOST=native)
• It builds as BMDA (make PROBE_HOST=hosted)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[esden]

LGTM