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)

(cherry picked from commit fb2f239ca1bd05f609e9e1e06b4b84f7f628d21b)