Add off_t, off32_t and off64_t types for Linux and Windows platforms

Currently `off64_t` is defined differently for each platform and thus it's not possible to use it in platform-independent way.

* i386-linux - `__off64_t`
* i386-windows - `_off64_t`
* x86_64-linux - `__off64_t`
* x86_64-windows `_off64_t`

With this PR it will be same for Linux and Windows platforms according to respective platform sizes.
In C `off_t` is defined as native long, thus 32-bit for i386 and 64-bit for x86_64. There's only exception when `_FILE_OFFSET_BITS=64` is defined when `off_t` will always be 64-bit.