arch/async_posix.h: improve portability.

{make|swap|get|set}context are removed in POSIX.1-2008, but glibc
apparently keeps providing it.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7420)

(cherry picked from commit 9d71a24e)