Add GCC+Makefiles

[zoobab]

Hi,

I have forked your project to add GCC+Makefiles here:

https://github.com/zoobab/STM32F103C8T6_CMSIS-DAP_SWO

I could compile most of it, but when it comes to the USB part, it fails.

I will continue to tomorrow to see if I can to have it compiled under GCC.