fix -u syntax

Author: goderbauer

pkgs/native_toolchain_c/lib/src/cbuilder/linker_options.dart

@@ -51,7 +51,7 @@ class LinkerOptions {
   }) : _linkerFlags = <String>[
          ...flags ?? [],
          '--strip-debug',
-         if (symbols != null) ...symbols.expand((e) => ['-u=$e']),
+         if (symbols != null) ...symbols.expand((e) => ['-u,$e']),
        ].toList(),
        gcSections = true,
        _wholeArchiveSandwich = symbols == null,

pkgs/native_toolchain_c/test/clinker/treeshake_helper.dart

@@ -21,7 +21,7 @@ Future<void> runTreeshakeTests(
     assetName: '',
     sources: sources,
     linkerOptions: LinkerOptions.manual(
-      flags: ['--strip-debug', '-u=my_other_func'],
+      flags: ['--strip-debug', '-u,my_other_func'],
       gcSections: true,
       linkerScript: packageUri.resolve(
         'test/clinker/testfiles/linker/symbols.lds',