Change call to mmap64.

The code was using an off64_t but calling mmap. This caused the code
to abort on 32 bit.

Add a unit test that would abort on the previous version.

Bug: 155662887

Test: New unit test passes.
Change-Id: I7a6efbc0d4227403c3d08a08deea56f239382157
Merged-In: I7a6efbc0d4227403c3d08a08deea56f239382157
(cherry picked from commit 7b9f35c9)