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/solaris_base.rs

@@ -6,7 +6,7 @@ pub fn opts() -> TargetOptions {
         dynamic_linking: true,
         executables: true,
         has_rpath: true,
-        os_family: Some("unix".to_string()),
+        families: vec!["unix".to_string()],
         is_like_solaris: true,
         limit_rdylib_exports: false, // Linker doesn't support this
         eh_frame_header: false,