Auto merge of #135030 - Flakebi:require-cpu, r=workingjubilee

Target option to require explicit cpu

Some targets have many different CPUs and no generic CPU that can be used as a default. For these targets, the user needs to explicitly specify a CPU through `-C target-cpu=`.

Add an option for targets and an error message if no CPU is set.

This affects the proposed amdgpu and avr targets.

amdgpu tracking issue: #135024
AVR MCP: rust-lang/compiler-team#800

Author: bors