Add support to emit VTR XML architecture file

[acomodi]

In the context of being able to support the interchange format in multiple P&R tools, the next good candidate would be enabling place and route starting from an interchange device using VTR.

With #85 that is about to land, we can start iterating over a tool to generate an XML out of the testarch, compatible with VTR.

This tool/script would be similar as to nextpnr_emit and populate_chip_info.