In #31686 changes were introduced to ensure that capacity was stored in
the ManagedBuffer allocation, and @lorentey sugested that as a stopgap
measure for addressing the lack of platform malloc introspection on
OpenBSD, we use Swift availability attributes instead on the relevant
parts of ManagedBuffer and friends.
Since platform availability symbols must be specifically set up to be
used, this commit does so in advance of the above change.
