GH-120024: Use pointer for stack pointer

[markshannon]

To support deferred reference counting, the stack pointer will need to be spilled around many calls.
The current implementation of the stack pointer as an offset from the 0th local will be too slow for that as each spill and reload will need to perform a calculation of the stack pointer from the stacktop or vice-versa.

This PR changes the stacktop offset to stackpointer pointer, so that no calculation is needed for the spill.

• Issue: Deferred reference counts #120024

[Fidget-Spinner]

Even without deferred refcounts, this is a simplification in my eyes (and also brings Tier 1 and Tier 2 representations closer together), so we should do this regardless.