[opcodes] ARC + PPC: Fix -Walloc-size warnings

Recently, -Walloc-size warnings started to kick in. Fix these two
calloc() calls to match the intended usage pattern.

opcodes/ChangeLog:

	* arc-dis.c (init_arc_disasm_info): Fix calloc() call.
	* ppc-dis.c (powerpc_init_dialect): Ditto.