Rollup merge of #129940 - liushuyu:s390x-target-features, r=RalfJung

s390x: Fix a regression related to backchain feature

In #127506, we introduced a new IBM Z-specific target feature, `backchain`.

This particular `target-feature` was available as a function-level attribute in LLVM 17 and below, so some hacks were used to avoid blowing up LLVM when querying the supported LLVM features.

This led to an unfortunate regression where `cfg!(target-feature = "backchain")` will always return true.

This pull request aims to fix this issue, and a test has been introduced to ensure it will never happen again.

Fixes #129927.

r? `@RalfJung`

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -353,9 +353,7 @@ pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
                 None
             }
         })
-        .filter(|feature| {
-            RUSTC_SPECIAL_FEATURES.contains(feature) || features.contains(&Symbol::intern(feature))
-        })
+        .filter(|feature| features.contains(&Symbol::intern(feature)))
         .map(|feature| Symbol::intern(feature))
         .collect()
 }