Support call stack spilling for unprotected outcalls

[jovanbulck]

Regarding inter-SM function calls, sancus-cc currently only supports up to four 16-bit arguments to be passed in CPU registers. As such, we do not support inter-SM calls that involve spilling function arguments on the callee's call stack.

In case an SM calls the unprotected domain (e.g., printf), however, the compiler should be able to generate code that spills arguments on the unprotected call stack (i.e., using __unprotected_sp).