Do not use 64-bit file IO calls in mmfile
Do not use 64-bit file IO calls in mmfile

These functions are not available on iOS or Android.

If libbitcoin is being compiled for a 64-bit platform, the standard
functions already work fine (size_t and off_t are already 64-bits).

If libbitcoin is being compiled for a 32-bit platform, files larger than
64GiB won't fit in the address space anyhow, so there is no point in being
64-bit aware here.

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