RFC: C11 (or C17) requirement

[carlescufi]

With the C language evolving, there are a few features in ISO/IEC 9899:2011 (aka C11) that would be useful to Zephyr developers. Given that compiler support for it seems to be already widespread, I'd like to start the conversation to discuss whether switching to it as a requirement to build Zephyr is something we would like to consider.
We could instead require C17, given that it is functionally equivalent to C11 but with some issues addressed.

Please use this issue to describe both features from C11 that you'd like to use and objections or showstoppers that you can foresee if we moved forward with the transition.

[nordic-krch]

It seems that logger would benefit greatly from _Generic feature added in C11. It allows to evaluate at compile time type of the parameter allowing to detect at compile time:

• potential strings (char *) which would allow to do internal string duplication without need of explicit log_strdup call which was always a problem
• floats for which double space needs to be allocated.
• logging 64 bit values

[pabigot]

Detecting multi-word arguments should already be handled in cbprintf, but log_strdup() is a problem. Though I'm not clear how _Generic could help with a varargs API like logging.

Browsing the C11 additions:

• Using standard thread APIs could be useful. People clearly want that in C++ for Zephyr, and the C semantics should be compatible.

A consideration is that MISRA is not yet past C99.