Fix libbitcoin build errors for iOS, OS X, and Android

With these two patches, libbitcion builds again for mobile platforms.

The 64-bit file functions are non-POSIX, and seem to only exist on Linux for legacy reasons. The modern standard is to use the normal functions (stat, open, mmap, etc.), but `#define _FILE_OFFSET_BITS 64` when compiling on 32-bit platforms to get the 64-bit behaviour. In our case, we don't even need that, since a 32-bit platform can't mmap a 64-bit file in the first place.