Ensure that the default codegen backend is set

Author: GuillaumeGomez

build_system/src/test.rs

@@ -588,6 +588,8 @@ fn asm_tests(env: &Env, args: &TestArg) -> Result<(), String> {
             &"0",
             &"--set",
             &"build.compiletest-allow-stage0=true",
+            &"--set",
+            &"rust.codegen-backend=[\"gcc\"]",
             &"tests/assembly-llvm/asm",
             &"--compiletest-rustc-args",
             &rustc_args,
@@ -1071,6 +1073,8 @@ where
             &"0",
             &"--set",
             &"build.compiletest-allow-stage0=true",
+            &"--set",
+            &"rust.codegen-backend=[\"gcc\"]",
             &format!("tests/{test_type}"),
             &"--compiletest-rustc-args",
             &rustc_args,