Johann's Standard Library
Johann's standard library is minimal. Functions are grouped by the file defining them, which is currently an opaque detail.
If you want to use header files for the standard library, there is a jstdlib.jnh next to jstdlib.o, ready to go.
Johann vs Assembly
While the compiler is written entirely in Johann itself, parts of the standard library are still implemented in assembly. System calls can't be made with Johann syntax directly, which motivates most of the lingering assembly. This is some of the oldest code, and hasn't been rewritten.
The printf and syscall functions are variadic, which Johann doesn't yet have support for, so they must remain assembly. printf has a special exemption from --strict for this reason. syscall probably should as well; I'll add one if/when it proves useful enough.