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.

compiler/rustc_codegen_ssa/src/base.rs

@@ -616,6 +616,11 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
         return ongoing_codegen;
     }
 
+    if tcx.sess.target.need_explicit_cpu && tcx.sess.opts.cg.target_cpu.is_none() {
+        // The target has no default cpu, but none is set explicitly
+        tcx.dcx().emit_fatal(errors::CpuRequired);
+    }
+
     let cgu_name_builder = &mut CodegenUnitNameBuilder::new(tcx);
 
     // Run the monomorphization collector and partition the collected items into

compiler/rustc_codegen_ssa/src/errors.rs

@@ -491,6 +491,10 @@ pub(crate) struct CheckInstalledVisualStudio;
 #[diag(codegen_ssa_insufficient_vs_code_product)]
 pub(crate) struct InsufficientVSCodeProduct;
 
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_cpu_required)]
+pub(crate) struct CpuRequired;
+
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_processing_dymutil_failed)]
 #[note]