Cleanup vxworks support to be able to compile for VxWorks 7

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7569)

(cherry picked from commit 5c8b7b4c)