VMS: compensate for gmtime_r() parameter pointer size

With VMS C, the second parameter takes a 32-bit pointer.  When
building with 64-bit pointer size default, we must compensate.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2811)
(cherry picked from commit 48ce800a)