kernel: thread: Add casting to pointers in the log message

Using char pointers for %p should be avoided in log messages. It will
cause issues in configurations where logging strings are removed from
the binary and they are not inspected when cbprintf packages from
logging string are built. In that case any char pointers are treated as
strings and copied into the pacakge body.

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