Fix to new executable allocation code (fixed print002 etc.)

Fix to new executable allocation code (fixed print002 etc.)
The problem here is caused by the fact that info tables include a
relative offset to the string naming the constructor.  Executable
memory now resides at two places in the address space: one for writing
and one for executing.  In the info tables generated by GHCi, we were
calculating the offset relative to the writable instance, rather than
the executable instance, which meant that the GHCi debugger couldn't
find the names for constructors it found in the heap.