Reverted your commit

Simon I reverted your commit https://phabricator.haskell.org/rGHC546f24e4f8a7c086b1e5afcdda624176610cbcf8 because it made every single binary that GHC builds seg-fault on windows. Even main = return () seg-faults. You may want to review and re-commit it. Simon commit 24864ba5587c1a0447beabae90529e8bb4fa117a Author: Simon Marlow Date: Sat Apr 23 22:14:43 2016 +0100 Use __builtin_clz() to implement log_2() A microoptimisation in the block allocator.