README.Windows: MinGW-w64 is known to work for building, regular MinGW is quirky

README.Windows: MinGW-w64 is known to work for building, regular MinGW is quirky

The regular MinGW doesn't work properly because it doesn't try to
provide C99 compatibility currently.

Change-Id: I27c1b9e1496a8c32032fab08a29cbe1124316edd
Reported-by: Tomáš Voda <Voda.Tomas@seznam.cz>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1638
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>