VMS: only use _realloc32 with /POINTER_SIZE=32

This fixes the following error when building with no particular pointer size
is specified (implied 32 bit):

    static void *(*realloc_func) (void *, size_t) = realloc;
    ................................................^
    %CC-E-UNDECLARED, In the initializer for realloc_func, "_realloc32" is not declared.
    at line number 93 in file DEV:[OPENSSL102.crypto]mem.c;1

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