Ignore linker warnings on macOS for ui-fulldeps

ld is showing things like this:
```
ld: ignoring duplicate libraries: '-lm'
```

I don't have time or a macbook that lets me investigate these. Just silence them for now.

Author: jyn514

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

@@ -1862,6 +1862,10 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         let mut hostflags = flags.clone();
         hostflags.push(format!("-Lnative={}", builder.test_helpers_out(compiler.host).display()));
         hostflags.extend(linker_flags(builder, compiler.host, LldThreads::No));
+        // FIXME: on macOS, we get linker warnings about duplicate `-lm` flags. We should investigate why this happens.
+        if suite == "ui-fulldeps" && target.ends_with("darwin") {
+            hostflags.push("-Alinker_messages".into());
+        }
         for flag in hostflags {
             cmd.arg("--host-rustcflags").arg(flag);
         }