add "v5" target env armv7a-vex-v5 target spec

Author: Tropix126

compiler/rustc_target/src/spec/targets/armv7a_vex_v5.rs

@@ -18,6 +18,7 @@ pub(crate) fn target() -> Target {
             os: "vexos".into(),
             is_like_vexos: true,
             vendor: "vex".into(),
+            env: "v5".into(),
             cpu: "cortex-a9".into(),
             abi: "eabihf".into(),
             features: "+v7,+neon,+vfp3,+thumb2".into(),