Linker error using Atmel Studio with C++ project
You mention Atmel's SMART ARM-based Microcontroller series D. Since these errors are appearing at compilation, the issue is unlikely to be limited to the series. I always search first for the error rather than for examples for a particular microcontroller. Manufacturer sample projects are better as a jumping-off point than an explanation of a problem, and there may not be much posted by others.

So, focusing on the error - a search finds the same errors on a different model, in a different environment. Also here. System calls (_sbrk) that would be defined in a Unix system may not be defined on a microcontroller, so yes, you need to add the definitions. Atmel has listed possibilities (#10): the rdimon and nosys libraries. Getting nosys into the project appears to solve the problem.