[gn] Rebase paths in symlink_or_copy against root_build_dir

We should be always rebasing paths against root_build_dir which is
the directory where scripts are run from, not root_out_dir which is
the current toolchain directory. The latter can result in invalid
paths when the action is being used from a non-default toolchain.

Differential Revision: https://reviews.llvm.org/D60330

llvm-svn: 357798

llvm/utils/gn/build/symlink_or_copy.gni

@@ -16,9 +16,9 @@ template("symlink_or_copy") {
     script = "//llvm/utils/gn/build/symlink_or_copy.py"
     args = [
       "--stamp",
-      rebase_path(stamp, root_out_dir),
+      rebase_path(stamp, root_build_dir),
       invoker.source,
-      rebase_path(invoker.output, root_out_dir),
+      rebase_path(invoker.output, root_build_dir),
     ]
   }
 }