Largely revert previous experiments with pointer sizes.

	* gdb/avr-tdep.c: Commenting tidied up.
	(avr_strip_flag_bits): Deleted.
	(avr_make_code_addr): Simplified
	(avr_make_code_ptr): #ifdef'd out
	(avr_make_data_addr): Simplified
	(avr_make_data_ptr): #ifdef'd out
	(avr_address_to_pointer): Comprehensive explanation in
	comments. Simplified and added sanity checks.
	(avr_pointer_to_address): Simplified and added sanity checks.
	(avr_integer_to_address): Simplified and better commenting.
	(avr_write_pc): Mask out flag bits when writing PC.
	(avr_unwind_pc): Add debug of unwound value and correct masking.
	(avr_dummy_id): Add code flags to PC in frame build.
	(avr_push_dummy_call): Remove MSBs from return_pc, make data
	adddress when writing SP.
	(avr_gdbarch_init): Return pointer and address bit sizes to
	upstream values.