shrink TOKENEXTRA and vp8_extra_bit_struct

Per John's previous change, shrink TOKENEXTRA from 20 to 8 bytes
original: b7b1e6fb
reverted: 41f4458a

Also drop unused field from vp8_extra_bit_struct

Update ARM ASM to deal with this change. In particular, Extra is signed
and needs to be sign-extended when loaded.

Change-Id: Ibd0ddc058432bc7bb09222d6ce4ef77e93a30b41