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