Improve linker-flavor detection

Linker drivers such as gcc, clang or lld often have a version postfix,
e.g clang-12. The previous logic would not account for this and would
fall back to guessing the linker flavor to be the default linker flavor
for the target, which causes linker errors when this is not the case.
By accounting for the possible version postfix and also considering
g++ and clang++, we considerably reduce the amount of times the
fallback guess has to be used.

To simplify matching check for a version postfix and match against the
linker stem without any version postfix.
In contrast to gcc, clang supports all architectures in one binary.
This means there are no variants like `aarch64-linux-gnu-clang` and
there is no need to check for `-clang` variants.

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1231,12 +1231,21 @@ pub fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
                     sess.emit_fatal(errors::LinkerFileStem);
                 });
 
+                // Remove any version postfix.
+                let stem = stem
+                    .rsplit_once('-')
+                    .and_then(|(lhs, rhs)| rhs.chars().all(char::is_numeric).then_some(lhs))
+                    .unwrap_or(stem);
+
+                // GCC can have an optional target prefix.
                 let flavor = if stem == "emcc" {
                     LinkerFlavor::EmCc
                 } else if stem == "gcc"
                     || stem.ends_with("-gcc")
+                    || stem == "g++"
+                    || stem.ends_with("-g++")
                     || stem == "clang"
-                    || stem.ends_with("-clang")
+                    || stem == "clang++"
                 {
                     LinkerFlavor::from_cli(LinkerFlavorCli::Gcc, &sess.target)
                 } else if stem == "wasm-ld" || stem.ends_with("-wasm-ld") {