Load all builtin targets at once instead of one by one

This should give us some performance improvements as we won't need to
do the lookup for the _currently_ 287 targets we have.

compiler/rustc_target/src/spec/mod.rs

@@ -1662,6 +1662,14 @@ macro_rules! supported_targets {
             Some(t)
         }
 
+        fn load_all_builtins() -> impl Iterator<Item = Target> {
+            [
+                $( targets::$module::target, )+
+            ]
+            .into_iter()
+            .map(|f| f())
+        }
+
         #[cfg(test)]
         mod tests {
             // Cannot put this into a separate file without duplication, make an exception.
@@ -3360,6 +3368,11 @@ impl Target {
         }
     }
 
+    /// Load all built-in targets
+    pub fn builtins() -> impl Iterator<Item = Target> {
+        load_all_builtins()
+    }
+
     /// Search for a JSON file specifying the given target tuple.
     ///
     /// If none is found in `$RUST_TARGET_PATH`, look for a file called `target.json` inside the