Changes to ghc

Changes to ghc:
commit 7fa0b43593644fba8a3a60e5503a55268578d3c0
Author: Edward Z. Yang <ezyang@cs.stanford.edu>
Date:   Sun Apr 13 01:09:40 2014 -0700

    Make BlockAlloc.c comment slightly more accurate (fixes #8491)

    Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>