VMS: Disable the warning MAYLOSEDATA3

The warning MAYLOSEDATA3 is one you will always get when compiling
source that calculates the difference between two pointers with
/POINTER_SIZE=64.

The reason is quite simple, ptrdiff_t is always a 32-bit integer
regardless of pointer size, so the result of 'ptr1 - ptr2' can
potentially be larger than a 32-bit integer.  The compiler simply
warns you of that possibility.

However, we only use pointer difference within objects and strings,
all of them well within 2^32 bytes in size, so that operation is
harmless with our source, and we can therefore safely turn off that
warning.

Reviewed-by: Rich Salz <rsalz@openssl.org>