fix(cmd): locate GOROOT when the build is trimmed

Assisted-by: GLM 5.3 Flash

cmd/gasm/asmhdr_test.go

@@ -285,6 +285,24 @@ func TestGoAsmHeaderResolved(t *testing.T) {
 	}
 }
 
+// TestResolveGOROOT holds the two resolutions the header generator leans
+// on: a baked root that is a real directory passes through, and a missing
+// one falls back to what go env GOROOT names, which is what a -trimpath
+// build needs.
+func TestResolveGOROOT(t *testing.T) {
+	out, err := exec.Command("go", "env", "GOROOT").Output()
+	if err != nil {
+		t.Skip("go env GOROOT unavailable:", err)
+	}
+	root := strings.TrimSpace(string(out))
+	if got := resolveGOROOT(root); got != root {
+		t.Fatalf("a real baked root must pass through, got %q", got)
+	}
+	if got := resolveGOROOT(filepath.Join(root, "nonexistent")); got != root {
+		t.Fatalf("a missing baked root must resolve to %q, got %q", root, got)
+	}
+}
+
 func TestOtherGOOSFile(t *testing.T) {
 	for path, want := range map[string]bool{
 		"/x/sys_windows_amd64.s": true,