clang-cl relies on this env var being set for 2017 to find the linker.

Bug: 772123
Change-Id: Id03896504d38dc706b2bd96a2c3834c6cb9db9fe
TBR=brucedawson
Reviewed-on: https://chromium-review.googlesource.com/709697
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>