Allow only TRACE, DEBUG, and INFO as the build-time, minimal log level

As there's no noticeable performance difference between having the INFO
log level and the FATAL log level as the build's minimal log level,
force it to be TRACE, DEBUG, or INFO. This means that INFO, WARN, ERROR,
and FATAL logging is always enabled, whatever the build configuration.

This seems reasonable and could prove useful to support/debugging
operations.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ie4261fd16d344587ef4b37ea1715d7302ad25087
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1590