Allow setting target_family to multiple values

This enables us to set more generic labels shared between targets. For
example `target_family="wasm"` across all targets that are conceptually
"wasm".

See https://github.com/rust-lang/reference/pull/1006

compiler/rustc_target/src/spec/wasm32_unknown_emscripten.rs

@@ -38,7 +38,7 @@ pub fn target() -> Target {
         is_like_emscripten: true,
         panic_strategy: PanicStrategy::Unwind,
         post_link_args,
-        os_family: Some("unix".to_string()),
+        families: vec!["unix".to_string()],
         ..options
     };
     Target {