Reverted your commit

[Simon Peyton Jones]

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 <marlowsd at gmail.com>
Date:   Sat Apr 23 22:14:43 2016 +0100
    Use __builtin_clz() to implement log_2()
        A microoptimisation in the block allocator.
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