toolchain_build: Drop unused hosts and target
toolchain_build: Drop unused hosts and target

The --target=i686-nacl "ng" toolchain is never used and won't be in the
future.  The builds for arm-linux hosts are never used.  Drop these to
speed up the bot cycle time.

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

Review URL: https://codereview.chromium.org/1078853004

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