ART: Compress LengthPrefixedArray on 32-bit targets.

Previously, the LengthPrefixedArray<ArtMethod> on 32-bit
targets contained a 64-bit length field followed by the
ArtMethod elements with size only a multiple of 4, not 8.
Consequently, an odd-length array broke the alignment for
the following array which would have the 64-bit length
placed at an unaligned address.

To fix that, we make the length field 32-bit and explicitly
pass the alignment information to the LengthPrefixedArray.
This also makes the 32-bit boot image a bit smaller.
On Nexus 5, AOSP, ToT, the field section is 11528B smaller
and the method section is 21036B smaller. 64-bit targets
should see the same savings for the field section but no
difference for the methods section.

Change-Id: I3e03e7b94129025c8a1c117c27645a34dec516d2