infra: Add mixin for Goma arbitrary toolchain support

Also start using this mixin in some of the ATS builders.

Bug: 942651
Change-Id: I038b4e38d6d71d839013976e02292724947189ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534909
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Simon Que <sque@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643771}
1 file changed