make instrlist_insert_mov_immed_ptrsz() & friends available in drdecode

Several useful instruction generation routines are not available in drdecode: instrlist_insert_{mov,push}_{immed,instr}_ptrsz() . To make them available:

Move these from instrument.c to instrlist.c? And move the helpers from mangle_shared.c to new file mangle_utils.c? (Don't want to add mangle_shared.c to drdecode: don't really want */mangle.c either -- just a couple routines from there.) And expose the _arch inside */mangle.c?

As they're slightly higher-level and can involve multiple instructions they're not in instr*.c.