crt: Use native _fseeki64 symbol from msvcrt.dll on Windows Vista+

Function _fseeki64() is available in msvcrt.dll starting from Windows
Vista+. Change the emulation of the _fseeki64() function to use native
symbol when available via the msvcrt_or_emu_glue.h.

Signed-off-by: Martin Storsjö <martin@martin.st>