x86: rust: move unknown-to-rustc codegen features back to the targe…

…t spec file

Similar to c5eae3a ("x86: rust: move unknown-to-`rustc` codegen
features back to the target spec file"), but for `retpoline-external-thunk`:

    Only a subset of the LLVM codegen features are recognized by `rustc`,
    and since rust-lang/rust#87402 (Rust 1.61.0)
    the compiler gives a warning about it:

        warning: unknown feature specified for `-Ctarget-feature`: `mmx`
          |
          = note: it is still passed through to the codegen backend
          = note: consider filing a feature request

    ...since those features may be renamed or removed at any point:

        $ rustc --print target-features

        [...]

        Code-generation features cannot be used in cfg or #[target_feature],
        and may be renamed or removed in a future version of LLVM or rustc.

    Thus move them back to the target spec generated file.

    See rust-lang/rust#96472 as well for a report.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

arch/x86/Makefile

@@ -21,7 +21,6 @@ ifdef CONFIG_CC_IS_CLANG
 RETPOLINE_CFLAGS	:= -mretpoline-external-thunk
 RETPOLINE_VDSO_CFLAGS	:= -mretpoline
 endif
-RETPOLINE_RUSTFLAGS	:= -Ctarget-feature=+retpoline-external-thunk
 
 ifdef CONFIG_RETHUNK
 RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern
@@ -203,7 +202,6 @@ ifdef CONFIG_RETPOLINE
   ifndef CONFIG_CC_IS_CLANG
     KBUILD_CFLAGS += -fno-jump-tables
   endif
-  KBUILD_RUSTFLAGS += $(RETPOLINE_RUSTFLAGS)
 endif
 
 ifdef CONFIG_SLS

scripts/generate_rust_target.rs

@@ -204,7 +204,11 @@ fn main() {
             "data-layout",
             "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
         );
-        ts.push("features", "-3dnow,-3dnowa,-mmx,+soft-float");
+        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
+        if cfg.has("RETPOLINE") {
+            features += ",+retpoline-external-thunk";
+        }
+        ts.push("features", features);
         ts.push("llvm-target", "x86_64-linux-gnu");
         ts.push("target-pointer-width", "64");
     } else {