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)

TBR=fbarchard
Bug: 748501
Change-Id: Id8398ab5c4615c7c33dfa5ec793fdc8c0a717e57
Reviewed-on: https://chromium-review.googlesource.com/585307Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@google.com>