Build libbase with _FILE_OFFSET_BITS=64.

sehr points out that we already have two bugs caused by not doing this:
file.cpp has a call to pread with an off64_t that gets truncated, and
mapped_file.cpp does similar with mmap.

Bug: N/A
Test: ran tests
Change-Id: Ia098a836457e5adf109541235ad71c0ad7219da8