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 work file (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 the standard functions
still work fine.
Use the right constant for madvise

The POSIX_MADV_SEQUENTIAL constant is for posix_madvise, not madvise.

Also, this constant doesn't exist on Android.