Darwin: Fix a compiler warning

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13319 a5019735-40e9-0310-863c-91ae7b9d1cf9
Darwin: Make stack growth tracking consistent with other architectures

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13320 a5019735-40e9-0310-863c-91ae7b9d1cf9