making ./validate run tests on all CPUs by default

Good evening all! Currently when being ran './validate' script (without any parameters): - builds ghc using 2 parallel jobs - runs testsuite using 2 parallel jobs I propose to change the default value to amount of available CPUs: - build ghc using N+1 parallel jobs - run testsuite using N+1 parallel jobs Pros: - first-time users will get faster ./validate - seasoned users will need less tweaking for buildbots Cons: - for imbalanced boxes (32 cores, 8GB RAM) it might be quite painful to drag box out of swap What do you think about it? Actual patch: https://phabricator.haskell.org/D146 Thanks! -- Sergei