fix(goobj): external package and symbol indices and arm64 pair relocations

Assisted-by: GLM 5.3

asm/goobj_resolve_test.go

@@ -56,8 +56,12 @@ func TestResolveExternalSymbols(t *testing.T) {
 	if err != nil {
 		t.Fatalf("resolveExternalGOOBJ: %v", err)
 	}
-	if len(pkgIdx) != 1 || pkgIdx["runtime"] != 0 {
-		t.Errorf("pkgIdx = %v, want runtime→0", pkgIdx)
+	if len(pkgIdx) != 1 || pkgIdx["runtime"] != 1 {
+		// Index 0 is the dummy invalid package in the blkPkgIdx table;
+		// the loader's reader loop starts at 1 (cmd/link/internal/
+		// loader/loader.go: "PkgIdx 0 is a dummy invalid package"), so
+		// the first real package must carry index 1.
+		t.Errorf("pkgIdx = %v, want runtime→1", pkgIdx)
 	}
 	if _, ok := symIdx["runtime·g0"]; !ok {
 		t.Errorf("symIdx missing runtime·g0, got %v", symIdx)