Fix kernel codehash discrepancy

Attempt at fixing the Kernel hash discrepancy across runs.

How to reproduce it

Print the KERNEL code hash in say, prove_with_outputs(), and then run:

for i in {0..10}; do cargo test --release --test simple_transfer; done

Error

Upon inspection of the different code segments per parsed file, it appears that the inconsistency was coming from hex_prefix.asm, more particularly on the %stack macro which for some reasons seems to not be deterministic. I've replaced them by hardcoding what the optimizer had deduced.

On this branch, I've ran the above command 40 times and got the same hash every time.

Other changes

I also:

changed a variable name, that was identical to a label and could have messed things up
removed the duplicate copy of the kernel code in initialize_kernel_code() as this is already being done when initializing GenerationState

0xPolygonZero / plonky2