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: https://github.com/rust-lang/compiler-team/issues/800