doc: coding_guidelines: Add a MISRA-C mandatory rule to Zephyr

This guideline address three undefined behaviors listed in the C99
standard. Although Zephyr does not support/implement these functions,
they are part of the C library and could be implemented in the future.

Also this is mandatory rule in MISRA-C and if one day Zephyr decides to
claim compliance with MISRA-C this guideline has to be followed.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>