Use the installed/downloaded objdump instead of work-dir one.

Sometimes the work dir is empty and only cached results are used.
I couldn't switch the LLVM one from work-dir to install/download-dir
because we prune all the .a file in the download-dir.

BUG=none
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/983533003