Rework syscalls.c once system prints are implemented

[cathales]

#1857 introduced two new functions:

https://github.com/openhwgroup/core-v-verif/blob/1eac965d073abc18ab6aff5e6f62fd635b060746/cva6/tests/custom/common/syscalls.c#L78-L81

and

https://github.com/openhwgroup/core-v-verif/blob/1eac965d073abc18ab6aff5e6f62fd635b060746/cva6/tests/custom/common/syscalls.c#L164-L173

We might want to move or modify them once system prints is made via UART both in Spike and CVA6. (Related: openhwgroup/cva6#1421)

[MikeOpenHWGroup]

@cathales and @zchamski, what is the status of this Issue?

[zchamski]

This is a topic still open as the main focus remains on verification, leaving core-environment interactions at lower priority.