If a build uses multiple toolchain, some generator runs with multiple toolchain.

e.g. built targets
https://chromium-build-stats.appspot.com/ninja_log/2018/02/15/slave606-c4/ninja_log.slave606-c4.chrome-bot.20180214-210448.16229.gz/table?dedup=true
from https://ci.chromium.org/buildbot/tryserver.chromium.android/android_n5x_swarming_rel/362042

6927 outputs in android_clang_arm/gen directory (generated by secondary toolchain)
8433 outputs in gen directory (generated by default toolchain)

In most cases, generated code is architecture independent. So most of generation steps for secondary toolchain are not necessary steps.

Similar things happen on Nacl toolchains.
I will remove such duplication.