Add modtime implementation for mingw

Question: since the module definition is now platform-agnostic, the source could be wrapped in a ```#if MICROPY_ENABLE_MOD_TIME``` and modtime.c can be added to py? This seems cleaner and other platforms can then benefit from it as well instead of repeating the module and argument getting code - although it seems stmhal for instance only has sleep, so additional macros would be needed like MICROPY_MOD_TIME_HAS_TIME to enable clock() and time()?