liblog: use packed structs instead of raw unaligned reads

Per jmgao@, we still need to worry about unaligned integer accesses.

In any case, we already have the packed structs for all of the
unaligned data that we're reading, so this change favors using those
packed structs.

Bug: 142256213
Test: x86,arm32,arm64 liblog-unit-tests
Change-Id: I21fc629eac49895d03b5b31daa4cc494b0c4c230