use ssize_t/int64 and 64bit version of ftell on windows

[richinseattle]

On Windows, there is a separate API for ftell on 64bit, in addition due to long type being 32bit on Windows, switch to ssize_t

[karpathy]

Sorry can you say a bit more? Does the current code not work/run properly?

[richinseattle]

This was a fix for #165 This patch will successfully load llama2-7b after converting it.
On Windows the long type is 32bit and the ftell() api is only 32bit, limiting use to 2GB files.

On Windows, the distinct api for 64bit ftell is ftelli64() so that was a simple update. But then we need to use a type that is appropriate for 64bit sizes with possible -1 return value, so ssize_t makes sense. I updated the mmap code to use 64bit sizes which simplified some of the logic, but technically it will still work on 32bit as well my commit comment was a little misleading.