Display outputs for "executable" targets in "gn desc".

BUG=None

Review-Url: https://codereview.chromium.org/2105333002
Cr-Original-Commit-Position: refs/heads/master@{#403654}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4879ceddeebf12d55800f5bcb340c26e4f10fc10