I would (respectfully) disagree with bluenow and suggest that the bare-bones syscall.c be included in new Ac6 C/C++ projects. When I was starting out with SW4STM32, I ran into this problem too and eventually solved it on my own by looking at other complete project examples, eventually figuring out that this source file had to be part of my project. It was a bit frustrating though, and adds an extra hurdle for the “newbie” to get over.
The onus for removing or replacing syscalls.c with a more appropriate implementation for a more complex project, e.g. one that uses a RTOS or has more complex memory management requirements, should be on those people who, in all likelyhood, will be more advanced and experienced users and will know ahead of time that the default syscalls will need to be modified or replaced.