Sort out pseudo-registers and register types.

	* gdb/avr-tdep.c: Make X, Y and Z pseudo-regs, remove W (is it
	ever used) and make FP alias of Y, remove pseudo_pc.
	(struct gdbarch_tdep): Remove custom types, use builtin instead.
	(avr_register_name): Add pseudo regs.
	(avr_register_type): Add pseudo regs and use builtin types.
	(avr_pseudo_register_read): Rewritten for X, Y and Z.
	(avr_pseudo_register_write): Likewise.
	(avr_frame_prev_register): Remove reference to PSEUDO_PC_REGNUM.
	(avr_gdbarch_init): Don't set up custom types in tdep.
	(avr_dump_tdep): Remove types no longer in tdep.