ninja: put links into a pool

This uses the ninja "pool" feature to limit the number of concurrent
links to a reasonable number, rather than the current pessimistic-always-1
via flock.

R=iannucci@chromium.org, thakis@chromium.org
BUG=240314

Review URL: https://codereview.chromium.org/15817020

git-svn-id: http://gyp.googlecode.com/svn/trunk@1646 78cadc50-ecff-11dd-a971-7dbc132099af