sim/ppc: Fix linker error with -fno-common

GCC 10 enables -fno-common by default.  This resulted in a multiple
definition linker error since global variables were declared and defined
in a header file:

  ld: ld-insn.o:sim/ppc/ld-insn.h:221: multiple definition of
  `max_model_fields_len'; igen.o:sim/ppc/ld-insn.h:221: first defined here

sim/ppc

	* ld-insn.h (last_model, last_model_data, last_model_function,
	last_model_internal, last_model_macro, last_model_static):
	Delete.
	(max_model_fields_len, model_data, model_functions,
	model_internal, model_macros, model_static, models): Declare, but do not
	define.
	* ld-insn.c (last_model, last_model_data, last_model_function,
	last_model_internal, last_model_macro, last_model_static,
	max_model_fields_len, model_data, model_functions,
	model_internal, model_macros, model_static, models): Define.