lib: os: cbprintf: Fixes for c++ and coverity

Fixes for C++ includes:
- avoid calling static inline template function in static
assert on certain platforms which consider it non const expression
- Add 0 to variable before calling C++ argument storing function
to promote a variable.

Fix for coverity:
- avoid using sizeof(n + 0) by using local variable (gcc extension)

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