Use 64-bit version of stat(2) and mmap(2)

This is necessary to be able to work on large files (> 32-bit size)
on 32-bit platforms.

We don't have any test for these methods on large files, will add in
a follow-up change.

Bug: 27734593
Test: CtsLibcoreTestCases
Change-Id: I94b3f7f42c6b52dbd8345ae9837d67273dd18be2