Change cookbook to map target_arch to target_cpu

target_arch was listed as the gn equivalent of target_arch. This is
wrong.

BUG=570145

Review URL: https://codereview.chromium.org/1568623002

Cr-Original-Commit-Position: refs/heads/master@{#367912}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ff69eea6e2d53fab7acf55b3eb8cd554789feda2