Fix short linker error output

This PR does 2 things:

- It removes the braces when there's a single element. This is required since brace expansion (at
  least in bash and zsh) only triggers if there's at least 2 elements.

- It removes the extra `.rlib` suffixes of the elements. See
  https://github.com/rust-lang/rust/pull/135707#discussion_r2185212393 for context.

Running `cargo +stage1 build` on the following program:

```rust
unsafe extern "C" {
    fn foo() -> libc::c_int;
}

fn main() {
    let x = unsafe { foo() } as u32;
    // println!("{}", data_encoding::BASE64.encode(&x.to_le_bytes()));
}
```

Gives the following diff before and after the PR:

```diff
-/tmp/foo/target/debug/deps/{liblibc-faf416f178830595.rlib}.rlib
+/tmp/foo/target/debug/deps/liblibc-faf416f178830595.rlib
```

Running on the same program with the additional dependency, we get the following diff:

```diff
-/tmp/foo/target/debug/deps/{liblibc-faf416f178830595.rlib,libdata_encoding-84bb5aadfa9e8839.rlib}.rlib
+/tmp/foo/target/debug/deps/{liblibc-faf416f178830595,libdata_encoding-84bb5aadfa9e8839}.rlib
```

Do we want to add a UI test?

compiler/rustc_codegen_ssa/src/errors.rs

@@ -457,7 +457,7 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for LinkingFailed<'_> {
                 } else if arg.as_encoded_bytes().ends_with(b".rlib") {
                     let rlib_path = Path::new(&arg);
                     let dir = rlib_path.parent().unwrap();
-                    let filename = rlib_path.file_name().unwrap().to_owned();
+                    let filename = rlib_path.file_stem().unwrap().to_owned();
                     if let Some(ArgGroup::Rlibs(parent, rlibs)) = args.last_mut() {
                         if parent == dir {
                             rlibs.push(filename);
@@ -471,7 +471,7 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for LinkingFailed<'_> {
                     args.push(ArgGroup::Regular(arg));
                 }
             }
-            let crate_hash = regex::bytes::Regex::new(r"-[0-9a-f]+\.rlib$").unwrap();
+            let crate_hash = regex::bytes::Regex::new(r"-[0-9a-f]+").unwrap();
             self.command.args(args.into_iter().map(|arg_group| {
                 match arg_group {
                     // SAFETY: we are only matching on ASCII, not any surrogate pairs, so any replacements we do will still be valid.
@@ -494,7 +494,11 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for LinkingFailed<'_> {
                             Err(_) => false,
                         };
                         let mut arg = dir.into_os_string();
-                        arg.push("/{");
+                        arg.push("/");
+                        let needs_braces = 2 <= rlibs.len();
+                        if needs_braces {
+                            arg.push("{");
+                        }
                         let mut first = true;
                         for mut rlib in rlibs {
                             if !first {
@@ -513,7 +517,10 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for LinkingFailed<'_> {
                             }
                             arg.push(rlib);
                         }
-                        arg.push("}.rlib");
+                        if needs_braces {
+                            arg.push("}");
+                        }
+                        arg.push(".rlib");
                         arg
                     }
                 }