Implement GetDefaultConcurrentLinks() for Linux Builds.

This method is used to determine the size of the Ninja pool
which is used for links during the build. It is currently size 1 for Linux, because this method is not implemented.
After this patch it will be <total_memory> / 8Gb.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/23242012

git-svn-id: http://gyp.googlecode.com/svn/trunk@1701 78cadc50-ecff-11dd-a971-7dbc132099af