Disable multithreading for gold

gold is frequently crashing on the buildbots which is most likely due to a race
condition when running in multithreaded mode. Disable multithreaded mode
until the bug is fixed.

BUG=161942
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/137573003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244811 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244811