Hi, if the C++ front-end decides that something will need constructing,
it will silently put the stuff into .rodata so that according
pgm_read_xxx will read garbage from .progmem.

As proposed by Jason, this patch diagnoses such situations.

Ok to commit?

Johann

	PR target/81407
	* config/avr/avr.c (avr_encode_section_info)
	[progmem && !TREE_READONLY]: Error if progmem object needs
	constructing.