Fix target_env in avr-unknown-gnu-atmega328

The target name itself contains GNU, we should set that in the
environment as well.

compiler/rustc_target/src/spec/base/avr_gnu.rs

@@ -19,6 +19,8 @@ pub(crate) fn target(target_cpu: &'static str, mmcu: &'static str) -> Target {
         llvm_target: "avr-unknown-unknown".into(),
         pointer_width: 16,
         options: TargetOptions {
+            env: "gnu".into(),
+
             c_int_width: "16".into(),
             cpu: target_cpu.into(),
             exe_suffix: ".elf".into(),