add support for ./x check src/tools/linkchecker

Author: pietroalbini

src/bootstrap/src/core/build_steps/check.rs

@@ -616,3 +616,9 @@ tool_check_step!(CoverageDump {
     mode: Mode::ToolBootstrap,
     default: false
 });
+
+tool_check_step!(Linkchecker {
+    path: "src/tools/linkchecker",
+    mode: Mode::ToolBootstrap,
+    default: false
+});

src/bootstrap/src/core/builder/mod.rs

@@ -1033,6 +1033,7 @@ impl<'a> Builder<'a> {
                 check::Compiletest,
                 check::FeaturesStatusDump,
                 check::CoverageDump,
+                check::Linkchecker,
                 // This has special staging logic, it may run on stage 1 while others run on stage 0.
                 // It takes quite some time to build stage 1, so put this at the end.
                 //