valgrind: Fix darwin build

The bzero-patch was merged upstream in
git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16103, so it does no
longer apply.

Additionally - to make the build succeed on darwin systems more recent
than our nixpkgs.darwin.xnu kernel version - we need to teach the build
the version of the xnu headers we provide, instead of letting the build
figure out the actual system version using `uname -r`.