We use an alignment prefix for unusual alignment kinds (not kWordAligned).
This will cause new objects to be aligned in ReadObject, and back references
to be aligned to skip padding.

The motivation to change the undefined next sentinel in WeakCell is this:
When the deserializer aligns an object, it requires filler maps to already
exist to create filler objects as padding. However, deserializing the
filler map leads to deserializing NaN, which as heap number is aligned:
filler map > meta map > weak cell cache > undefined > NaN
If we use the-hole instead of undefined as sentinel, it works.

R=jochen@chromium.org,bbudge@chromium.org
BUG=v8:4178
LOG=N

Review URL: https://codereview.chromium.org/1179873003

Cr-Commit-Position: refs/heads/master@{#29044}