AVR: target/121277 - Don't load 0x800000 with const __flashx *x = NULL.

AVR: target/121277 - Don't load 0x800000 with const __flashx *x = NULL.

Converting from generic AS to __flashx used the same rule like
for __memx, which tags RAM (generic AS) locations by setting bit 23.
The justification was that generic isn't a subset of __flashx, though
that lead to surprises with code like const __flashx *x = NULL.

The natural thing to do is to just load 0x000000 in that case,
so that the null pointer works in __flashx as expected.

Apart from that, converting NULL to __flashx (or __flash) no more
raises a -Waddr-space-convert diagnostic.

gcc/
	PR target/121277
	* config/avr/avr.cc (avr_addr_space_convert): When converting
	from generic AS to __flashx, don't set bit 23.
	(avr_convert_to_type): Don't -Waddr-space-convert when NULL
	is converted to __flashx or to __flash.