Clean up toolchain paths
Clean up toolchain paths

Switch toolchain_build_bionic.py to use a single function to generate
a toolchain path.  Uses pynacl to determine OS name.

TEST=./toolchain_build_bionic.py
BUG=none
R=dyen@chromium.org

Review URL: https://codereview.chromium.org/203213007

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12897 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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