cbuildbot_config: use binary toolchains for mips

Now that the SDK builds these for us, we can start using prebuilts.

BUG=chromium:380329
TEST=`cbuildbot urara-release` passes
CQ-DEPEND=CL:226686

Change-Id: I9b3e85fa9b6c97619be71cb5ae9d9836bcdd47e6
Reviewed-on: https://chromium-review.googlesource.com/228272
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
2 files changed