windows: Sync mpconfigport.h with the unix' version

- rearrange/add definitions that were not there so it's easier to compare both
- use MICROPY_PY_SYS_PLATFORM in main.c since it's available anyway
- define EWOULDBLOCK, it is missing from ingw32
Merge pull request #730 from stinos/windows-mpconfig

windows: Sync mpconfigport.h with the unix' version