Fix target_env in avr-unknown-gnu-atmega328

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

Author: madsmtm

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(),