bootstrap: provide host `rust_test_helpers` to compiletest, not just target.

src/bootstrap/test.rs

@@ -978,6 +978,8 @@ fn run(self, builder: &Builder) {
 
         // HACK(eddyb) ensure that `libproc_macro` is available on the host.
         builder.ensure(compile::Test { compiler, target: compiler.host });
+        // Also provide `rust_test_helpers` for the host.
+        builder.ensure(native::TestHelpers { target: compiler.host });
 
         builder.ensure(native::TestHelpers { target });
         builder.ensure(RemoteCopyLibs { compiler, target });
@@ -1046,7 +1048,11 @@ fn run(self, builder: &Builder) {
             cmd.arg("--linker").arg(linker);
         }
 
-        let hostflags = flags.clone();
+        let mut hostflags = flags.clone();
+        hostflags.push(format!(
+            "-Lnative={}",
+            builder.test_helpers_out(compiler.host).display()
+        ));
         cmd.arg("--host-rustcflags").arg(hostflags.join(" "));
 
         let mut targetflags = flags;