Add large file support to create_file for 32-bit.
The tests need to create files larger than 2GB, but size_t is 32-bit
on a 32-bit system. Make use of explicit off64_t APIs so we can still
use a default off_t for the tests while enabling 64-bit file offsets
Reviewers: mclow.lists, EricWF
Reviewed By: EricWF
Subscribers: christof, ldionne, libcxx-commits
Differential Revision: https://reviews.llvm.org/D56619