toolchain_build: Use source targets for tarballs
toolchain_build: Use source targets for tarballs

Use 'source' targets for unpacking tarballs from third_party too.
After this, all of the old support for 'git_url' and 'tar_src' in
'build' targets can go away entirely.

BUG= none
TEST= toolchain trybots
R=bradnelson@google.com, dschuff@chromium.org

Review URL: https://codereview.chromium.org/110283004

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12520 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client