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_target/src/spec/mod.rs

@@ -2240,6 +2240,9 @@ pub struct TargetOptions {
     /// Default CPU to pass to LLVM. Corresponds to `llc -mcpu=$cpu`. Defaults
     /// to "generic".
     pub cpu: StaticCow<str>,
+    /// Whether a cpu needs to be explicitly set.
+    /// Set to true if there is no default cpu. Defaults to false.
+    pub need_explicit_cpu: bool,
     /// Default target features to pass to LLVM. These features overwrite
     /// `-Ctarget-cpu` but can be overwritten with `-Ctarget-features`.
     /// Corresponds to `llc -mattr=$features`.
@@ -2676,6 +2679,7 @@ impl Default for TargetOptions {
             link_script: None,
             asm_args: cvs![],
             cpu: "generic".into(),
+            need_explicit_cpu: false,
             features: "".into(),
             direct_access_external_data: None,
             dynamic_linking: false,