crt: Fix overflow detection in mingw-w64 _lseeki64() emulation

On recent Windows versions when the current file position does not fit into
the 32 bits then the _lseek() function returns error without setting new
position.

Reflect this behavior in _lseeki64() emulation and fallback to
SetFilePointer() on error.

This fixes t_lseeki64.c test when compiled against msvcrt20.dll and run on
Windows 10 against the system os version of msvcrt20.dll.