Run tests/rustdoc-json/attrs/target_features on all hosts.

Makes it more convenient to test rustdoc on non x86_64. I mainly care
about the aarch64 dev-desktops.

This works because rustdoc uses all target features, not just that of
the target.

tests/rustdoc-json/attrs/target_feature.rs

@@ -1,5 +1,3 @@
-//@ only-x86_64
-
 //@ is "$.index[?(@.name=='test1')].attrs" '["#[target_feature(enable=\"avx\")]"]'
 #[target_feature(enable = "avx")]
 pub fn test1() {}