Fix large file allocation for 32 bit unix

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File allocations over 2Gb that use the allocate method currently fail on 32 bit unix as libc::off_t is defined as 32 bit. Using posix_fallocate64 with a libc::off64_t fixes it.

The test requires that your tempdir has at least 2Gb free and can support files that are 2Gb in size. Not sure this is a great requirement, but not sure how else to test it.