The correct usage is to use root_out_dir in the output file name
and to use root_build_dir in the rebased arguments.

Fixes the 32-bit NACL build on Windows. (Tested locally)

Bug: dawn:283
Change-Id: I1d6ca062306a4662eef5c922296e9c95fb047acd
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38828Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Jamie Madill <jmadill@chromium.org>
Presubmit-Ready: Jamie Madill <jmadill@chromium.org>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>