Fix gn bootstrap build with --use-custom-libcxx

The gn binary gets copied up one directory when bootstrapping.  It was failing
to run since it couldn't find libc++.so which wasn't copied.  This CL copies
libc++.so too and renames it to libc++.gn.so to not conflict with Chromium's
libc++.so.

Also rename README.md to README.chromium.md to not conflict with gn's README.md.

BUG=807400
R=thakis

Change-Id: Idce60c7df7e8aaf134357f39347ee4dafe2a851e
Reviewed-on: https://chromium-review.googlesource.com/c/1474847
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632494}
3 files changed