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(),

tests/ui/check-cfg/well-known-values.stderr

@@ -156,7 +156,7 @@ warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
 LL |     target_env = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: expected values for `target_env` are: ``, `gnu`, `msvc`, `musl`, `newlib`, `nto70`, `nto71`, `ohos`, `p1`, `p2`, `psx`, `relibc`, `sgx`, and `uclibc`
+   = note: expected values for `target_env` are: ``, `gnu`, `msvc`, `musl`, `newlib`, `nto70`, `nto71`, `ohos`, `p1`, `p2`, `psx`, `relibc`, `sgx`, `uclibc`, and `v5`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`