GET STAT OF MYD FILE

Description: Impossible to neither REPAIR nor OPTIMIZE .MYD files bigger 
than 4GB. Only the Windows versions are affected.

Analysis: In my_copystat() function while calling stat() system call for
Windows it is unable to get the file information for files greater than
4GB. Hence stat() fails with error and OPTIMIZE table on 4GB ".MYD" files  
fails. And the corresponding .TMD file(temporary .MYD file) is not getting
deleted.

Fix: Now we are calling windows specific stat() system call i.e _stati64()
instead of stat() for windows and stat() call for other OS's.