Do not pass LLD flags to targets flags of compiletest

Not all targets support these flags, so we cannot just pass them to the tests unconditionally. Before, we were using a linker arg (`-Clink-arg=-fuse-ld=lld`), which circumvented this in a hacky way.

src/bootstrap/src/core/build_steps/test.rs

@@ -1901,7 +1901,6 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
 
         let mut targetflags = flags;
         targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
-        targetflags.extend(linker_flags(builder, compiler.host, LldThreads::No));
         for flag in targetflags {
             cmd.arg("--target-rustcflags").arg(flag);
         }