Support for m86k architecture (e.g: Sega Genesis) about gdbstub

[daniel5151]

Thank you for bringing this critical omission to light.

Based on some cursory research (you'll have to apologize - I'm clearly an uncultured programmer who lacks experience working with such an ikonic console 🎮), it seems the Sega Genesis is based around a Motorola 68000 (m68k) CPU, and as such, it should be relatively straightforward to add support for it in gdbstub via a custom Arch trait implementation.

The relevant target.xml file can be found here: https://github.com/bminor/binutils-gdb/blob/master/gdb/features/m68k-core.xml

If you are interested in seeing a m86k target upstreamed in gdbstub, I would be more than glad to upstream any PRs sent my way.

Again, I sincerely apologize for omitting this critical architecture from gdbstub_arch, and I do hope this issue can be closed in a timely manner.