Fix more DSA problems uncovered by the buildfarm.

Fix more DSA problems uncovered by the buildfarm.

On 32-bit systems, don't try to use 64-bit DSA pointers, because the
computation of DSA_MAX_SEGMENT_SIZE overflows Size.

Cast 1 to Size before shifting it, so that the compiler doesn't
produce a result of the wrong width.

In passing, change one use of size_t to Size.