[SOL] Optimize call convention in SBFv2

[LucasSte]

Problem

SBFv2 introduces dynamic frame sizes without stack gaps, so that we know during compile time the frame size for a function. As a consequence, the call convention inherited from SBFv1 in #84 becomes inefficient.

If there are more than five arguments in a function call in the old convention, we do:

1. Store four arguments in the registers.
2. Store the remaining arguments in the caller's frame, starting at the frame's higher address.
3. Copy the frame pointer to R5: mov r5, r10
4. Access the arguments in the callee using R5.

We are clobbering R5 unnecessarily in this scheme.

Solution

This PR introduces a new call convention for SBFv2, leveraging the dynamic stack frames.

In the new convention, we do the following:

1. Store five arguments in the registers (from r1 to r5).
2. Store the remaining arguments in the callee's frame, starting at the higher address.
3. Access the arguments in the callee using the frame pointer R10.

In this case, we free up one general purpose register and reduce one load and one store from function calls.

[dmakarov]

LGTM