GN: Fix CPU detection in Visual Studio generator

The previous version assumed that if there is no build override
for "current_cpu" then it means "x86".

R=brettw@chromium.org, tmoniuszko@opera.com
BUG=

Review URL: https://codereview.chromium.org/1733023002

Cr-Original-Commit-Position: refs/heads/master@{#377577}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5dd34fc85aab35792d7034ac3caf3a8f9f38ba8a
3 files changed
tree: 45152f22ff659a6c24039cca0b26fc85d12d052e
  1. tools/