Lack of architecture in gdbstub makes debugging confusing

I spent some quality time debugging GEF and came to a conclusion here:
https://github.com/hugsy/gef/issues/598#issuecomment-819174169

tldr;

* gdb_arch_name was undefined on riscv
* this bug was fixed recently via https://github.com/qemu/qemu/commit/edf647864bdab84ed4b1a4f47ea05be6bb075c69


* An undefined gdb_arch_name results in qemu's gdbstub omitting the <architecture> xml.
* gdb translates a missing <architecture> as "auto" which breaks a lot of stuff.
* tracking down where "auto" comes from is a bit confusing and time consuming.


It might be better to report a missing / blank gdb_arch_name as "<architecture>unknown</architecture>" instead of omitting the block completely.