These changes ensure that the packaged Windows SDK bits support targeting
ARM64 Windows. This change must not land until crrev.com/c/1339101 lands,
since it is needed to the varying number of vs_runtime_dll_dirs.

Bug: 893460
Change-Id: Ie32563067c6fb6078acfaccd6d3d572d1dd44888
Reviewed-on: https://chromium-review.googlesource.com/c/1330185
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>