Windows support

[MarekPikula]

It would be nice if this fork supported Windows. I know it's not your primary focus, but I guess it would enable wider audience to test VexRiscv and accompanying SoCs especially in corporate area, besides making it possible to merge it to upstream as requested in #8.

Right now I'm finishing my internship in Trinamic, which will probably use VexRiscv under Murax for some internal projects and maybe in future for some bigger projects. The problem is that we're using here mainly Windows. I had no problem making Murax work, since I'm one of those Linux guys here, but I needed to compile Windows version of OpenOCD for some other guys, what wasn't as simple as one might think.

I forked this project to include workarounds to make it compile under MinGW in some basic version here, but it's far from perfect. It's compiled without jtag-tcp, which uses sockets, since it's not directly required. It could be probably easily replaced with WinSock for Windows builds, as it was done in other OpenOCD modules (I think).