libfuzzer: Copy dicts to root_build_dir/ instead of target_out_dir/.
libfuzzer: Copy dicts to root_build_dir/ instead of target_out_dir/.

Previously, the dictionaries were being copied into obj/, which the
ClusterFuzz archive recipe explicitly ignores. This makes it
so that dictionaries are placed in out/Fuzzer/dicts/.

R=aizatsky@chromium.org,kcc@chromium.org

Review URL: https://codereview.chromium.org/1620483002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370869}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk