fully intercept cygwin malloc: sbrk, initial heap walk

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:58:26_

PR 595798

PR 476805 adds interception of malloc routines in every app library, but
the cygwin malloc needs special handling as it does not end up calling the
Rtl routines: sbrk is a library routine, and we'll need an initial heap
walk since we don't have early-enough injection.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=197_