GC changes

[dewar@gnat.com]

<<Actually there is another approach.  You could generate unwind stubs
similar to exception handling, with a table indexed by program counter
to the stubs.  For most program addresses the stub would be empty, but
for certain places where only an offset address is present, the stub
would generate the non-offset address.

This has the advantage that no optimisation is penalised.  It's quite
complex to implement though.
>>

Well it's a step along the route to type accurate GC, and really if you
even contemplate going this far, you may as well go all the way.