ninja/mac: Allow more than one ld process at a time, based on available RAM.

Allow one concurrent ld process for every 4 GB of ram. This matches what
ninja/windows does.

R=mark@chromium.org

Review URL: https://codereview.chromium.org/23622005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1706 78cadc50-ecff-11dd-a971-7dbc132099af