Refine the message to at least *mention* the attribute itself.

Update pre-existing test's diagnostic output accordingly.

src/librustc_codegen_llvm/consts.rs

@@ -116,7 +116,8 @@ fn check_and_apply_linkage(
         let llty2 = if let ty::RawPtr(ref mt) = ty.sty {
             cx.layout_of(mt.ty).llvm_type(cx)
         } else {
-            cx.sess().span_fatal(span, "must have type `*const T` or `*mut T`")
+            cx.sess().span_fatal(
+                span, "must have type `*const T` or `*mut T` due to `#[linkage]` attribute")
         };
         unsafe {
             // Declare a symbol `foo` with the desired linkage.

src/test/ui/linkage-attr/linkage2.rs

@@ -7,7 +7,7 @@
 
 extern {
     #[linkage = "extern_weak"] static foo: i32;
-    //~^ ERROR: must have type `*const T` or `*mut T`
+    //~^ ERROR: must have type `*const T` or `*mut T` due to `#[linkage]` attribute
 }
 
 fn main() {

src/test/ui/linkage-attr/linkage2.stderr

@@ -1,4 +1,4 @@
-error: must have type `*const T` or `*mut T`
+error: must have type `*const T` or `*mut T` due to `#[linkage]` attribute
   --> $DIR/linkage2.rs:9:32
    |
 LL |     #[linkage = "extern_weak"] static foo: i32;