posix_async: FreeBSD also defines {make|swap|get|set}context

FreeBSD also defines {make|swap|get|set}context for backward
compatibility, despite also exposing POSIX_VERSION 200809L in FreeBSD
15-current.

FreeBSD has defined these interfaces since FreeBSD 4.7, released over 20
years ago, so no further nuance in FreeBSD version number is necessary.

Pull Request: https://github.com/openssl/openssl/pull/23885
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