Update GN docs about target_cpu to encourage use of current_cpu

BUG=616819

Review-Url: https://codereview.chromium.org/2038733003
Cr-Original-Commit-Position: refs/heads/master@{#398661}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bfdee36d31a624d330b60b481fc26d10dff81b97
2 files changed
tree: 63c6f7c8513394dd97c0130cbffef0c67e7eda98
  1. tools/