Pass minicore path via bootstrap when building compiletest step

src/bootstrap/src/core/build_steps/test.rs

@@ -1731,6 +1731,9 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         cmd.arg("--run-lib-path").arg(builder.sysroot_libdir(compiler, target));
         cmd.arg("--rustc-path").arg(builder.rustc(compiler));
 
+        // Minicore auxiliary lib for tests that need std/core stubs in cross-compilation scenarios.
+        cmd.arg("--minicore-path").arg("tests/auxiliary/minicore.rs");
+
         let is_rustdoc = suite.ends_with("rustdoc-ui") || suite.ends_with("rustdoc-js");
 
         if mode == "run-make" {