Add metadata to targets

This adds four pieces of metadata to every target:
- description
- tier
- host tools
- std

This information is currently scattered across target docs and both
- not machine readable, making validation harder
- sometimes subtly encoding by the table it's in, causing mistakes and
  making it harder to review changes to the properties

By putting it in the compiler, we improve this. Later, we will use this
canonical information to generate target documentation from it.

compiler/rustc_target/src/json.rs

@@ -2,7 +2,9 @@ use std::borrow::Cow;
 use std::collections::BTreeMap;
 
 pub use serde_json::Value as Json;
-use serde_json::{Map, Number};
+use serde_json::{json, Map, Number};
+
+use crate::spec::TargetMetadata;
 
 pub trait ToJson {
     fn to_json(&self) -> Json;
@@ -120,3 +122,14 @@ impl ToJson for crate::abi::call::Conv {
         Json::String(s.to_owned())
     }
 }
+
+impl ToJson for TargetMetadata {
+    fn to_json(&self) -> Json {
+        json!({
+            "description": self.description,
+            "tier": self.tier,
+            "host_tools": self.host_tools,
+            "std": self.std,
+        })
+    }
+}