logging: support tagged argument in log message packaging

This adds support for packaging with tagged arguments for log
messages so that backends can make use of this information
when they process messages.

Note that currently tagged arguments must be used with
CONFIG_LOG2_ALWAYS_RUNTIME where log messages are created via
z_log_msg2_runtime_{v,}create(). For some reason, using C++ on
Cortex-M (mps2_an385) results in an additional empty string
argument being added to the list. This does not happen if only
doing C on mps2_an385, or on other architectures.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>