fix: Set allow_empty on lib_legacy filegroup (#636)

`lib_legacy` needs the same treatment `lib` got in
#635

Author: mortenmj

toolchain/BUILD.llvm_repo.tpl

@@ -103,7 +103,7 @@ filegroup(
         "lib/clang/{LLVM_VERSION}/lib/**",
         "lib/**/libc++*.a",
         "lib/**/libunwind.a",
-    ]),
+    ], allow_empty = True),
 )
 
 filegroup(