refactor: Move env parsing of deployment target to rustc_session

compiler/rustc_target/src/spec/base/apple/mod.rs

@@ -317,3 +317,15 @@ impl OSVersion {
         Self { major, minor, patch }
     }
 }
+
+/// Name of the environment variable used to fetch the deployment target on the given OS.
+pub fn deployment_target_env_var(os: &str) -> &'static str {
+    match os {
+        "macos" => "MACOSX_DEPLOYMENT_TARGET",
+        "ios" => "IPHONEOS_DEPLOYMENT_TARGET",
+        "watchos" => "WATCHOS_DEPLOYMENT_TARGET",
+        "tvos" => "TVOS_DEPLOYMENT_TARGET",
+        "visionos" => "XROS_DEPLOYMENT_TARGET",
+        _ => unreachable!("tried to get deployment target env var for non-Apple platform"),
+    }
+}