Fix link failure on AVR (incompatible ISA error) Fixes #137739. A reproducer of the issue is present there. I believe the root cause was introducing the avr-none target (which has no CPU by default) while also trying to get the ISA revision from the target spec. This commit uses the `target-cpu` option instead, which is already required to be present for the target. r? compiler cc ``@Patryk27``