Parallel recompilation is usually disabled on single-core systems.

R=jkummerow@chromium.org
BUG=v8:2733

Review URL: https://codereview.chromium.org/17261021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15231 ce2b1a6d-e550-0410-aec6-3dcde31c8c00