Add large file support to create_file for 32-bit.

Summary:
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
for create_file.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: christof, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D56619

llvm-svn: 351225