Fixes for NonStop builds on 3.5 to handle OS platform header file changes.

Fixes for NonStop builds on 3.5 to handle OS platform header file changes.

This changes handles the introduction of _POSIX_VERSION into the NonStop x86
header files that tricks OpenSSL into thinking that ucontext.h is available.

Fixes #28498

Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28769)