VMS: setbuf() only takes 32-bit pointers

Giving setbuf() a 64-bit pointer isn't faulty, as the argument is
passed by a 64-bit register anyway, so you only get a warning
(MAYLOSEDATA2) pointing out that only the least significant 32 bits
will be used.

However, we know that a FILE* returned by fopen() and such really is a
32-bit pointer (a study of the system header files make that clear),
so we temporarly turn off that warning when calling setbuf().

Reviewed-by: Andy Polyakov <appro@openssl.org>