Doing this makes binaries tinier, since we don't need to have all the
extra code for supporting a 32-bit address space. It also benefits us
because we're able to use WIN32 futexes, which makes locking simpler.
b69f3d2488 is what officially ended our
Windows 7 support. This change is merely a formalization. You can use
old versions of Cosmo now and forevermore if you need Windows 7 since
our repository is hermetic and vendors all its dependencies.
Won't fix#617
