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

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.

Note: There's no fallback for POSIX_VERSION 200809 without
these routines, so maybe that should be a #error?

CLA: Trivial

Sponsored by:		Netflix

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23885)