Remove align-16 attribute from ObjectData

Summary: Less coupling between the allocator and ObjectData. This constraint was
added so the jit could use movdqa to load and store TypedValues into
XMM registers, however we switched to movdqu a while ago. We don't have
evidence that using movdqu on an aligned address is any slower than
movdqa.

Reviewed By: @jdelong

Differential Revision: D1756285

Signature: t1:1756285:1419366514:035de81134fcd8cd10468b8f2dd9cafcd3eb1545