Fix a couple of bugs in the way info tables are generated for 64-bit platforms

1. The offset was a full word, but it should actually be a 32-bit
offset on 64-bit platforms.
2. The con_desc string was allocated separately, which meant that it
might be out of range for a 32-bit offset.

These bugs meant that +RTS -Di (interpreter debugging) would sometimes
crash on 64-bit.
Fix bugs in debug printing
Fix a couple of inaccurate stack checks
Disable an assertion; see comment
Add a comment about stack checks