Rollup merge of #137830 - LuigiPiucco:incompatible-isa-fix, r=workingjubilee

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``

Author: matthiaskrgr