clang packager/win: Check that GnuWin32 is in the path at the start.

It's needed to run tests, and I prefer getting an error message
immediately over getting one 17 minutes in, when the build is done
and tests start running.

BUG=82385

Review URL: https://codereview.chromium.org/1351143006

Cr-Commit-Position: refs/heads/master@{#349932}
1 file changed