i#1551 port to ARM: add OP_ldmib_priv and OP_stmib_priv

Adds two new opcodes OP_ldmib_priv and OP_stmib_priv which were incorrectly
listed as regular OP_ldm and OP_stm in the table.  Updates all the
associated pieces such as the encoding chain and the INSTR_CREATE_ macros.

Xref CL that discovered the missing opcodes: https://codereview.appspot.com/188470043/

Review-URL: https://codereview.appspot.com/186590043