Make global data const

Make global data const

Removes all runtime initialization of global data. This commit is a
squashed version of the following series cherry-picked from master.
This is necessary because of a change that was merged to the tester
that depends on the scaler being moved to the RTCD framework, which
is a worthwhile thing to include in Eider anyway.

  - a91b42f02 Makes all global data in entropy.c const
  - b35a0db0e Makes all global data in tokenize.c const
  - 441cac8ea Makes all mode token tables const
  - 5948a0210 Ports vpx_xcaler to new RTCD method
  - 317d4244c Makes all mode token tables const part 2

Change-Id: Ifeaea24df2b731e7c509fa6c6ef6891a374afc26