Implement GetDefaultConcurrentLinks() for Linux Builds.
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
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp