Simplify implementation of -Z gcc-ld

- The logic is now unified for all targets (wasm targets should also be supported now)
- Additional "symlink" files like `ld64` are eliminated
- lld-wrapper is used for propagating the correct lld flavor
- Cleanup "unwrap or exit" logic in lld-wrapper

compiler/rustc_codegen_ssa/src/back/command.rs

@@ -105,12 +105,7 @@ impl Command {
             }
             Program::Lld(ref p, flavor) => {
                 let mut c = process::Command::new(p);
-                c.arg("-flavor").arg(match flavor {
-                    LldFlavor::Wasm => "wasm",
-                    LldFlavor::Ld => "gnu",
-                    LldFlavor::Link => "link",
-                    LldFlavor::Ld64 => "darwin",
-                });
+                c.arg("-flavor").arg(flavor.as_str());
                 if let LldFlavor::Wasm = flavor {
                     // LLVM expects host-specific formatting for @file
                     // arguments, but we always generate posix formatted files