In cross builds of chrome/win, the host and target toolchains currently
have the same name.  To fix this, rename clang_x64 to win_clang_x64.
Because the toolchain name is also referenced in libyuv, this requires
a five-sided change:

1. Introduce variable containing the toolchain name in src.git
2. Change libyuv to refer to the variable
3. Rename toolchain in src.git (including in the newly introduced var)
4* Let libyuv refer to the new name directly
5. Remove variable again

(See also https://codereview.chromium.org/2463143002)

Bug: 748501
Change-Id: I89fdf1503f1a57992a8336026d4c8d767685d53f
Reviewed-on: https://chromium-review.googlesource.com/585306
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>