lib: os: cbprintf: Avoid using VLA

Some configurations does not support VLA (e.g. CONFIG_MISRA_SANE).
Replace with a trick were fixed size array is used.
Note that this code is resolved at compile time so code size and
stack usage should be the same.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>