PR34734 produces annoying, false warnings if __attribute__((progmem))
is used in conjunction with C++.  DECL_INITIAL is not yet set up in
avr_handle_progmem_attribute.

Johann

	PR target/34734
	* config/avr/avr.c (avr_handle_progmem_attribute): Move warning
	about uninitialized data attributed 'progmem' from here...
	(avr_encode_section_info): ...to this new function.
	(TARGET_ENCODE_SECTION_INFO): New define.
	(avr_section_type_flags): For data in ".progmem.data", remove
	section flag SECTION_WRITE.