Enable optimize max for arm in GN + update docs

docs/deprecated_builds.md

-# Getting Started
+# Deprecated Builds
 
-How to get and build the libyuv code.
+Older documentation on build configs which are no longer supported.
 
 ## Pre-requisites
 
 You'll need to have depot tools installed: https://www.chromium.org/developers/how-tos/install-depot-tools
 Refer to chromium instructions for each platform for other prerequisites.
 
 ## Getting the Code
 
 Create a working directory, enter it, and run:
 
@@ skipping 417 matching lines @@
     git checkout -b mycl -t origin/master
     git pull
     <edit files>
     git add -u
     git commit -m "my change"
     git cl lint
     git cl try
     git cl upload -r a-reviewer@chomium.org -s
     <once approved..>
     git cl land