core: Don't generate logic for log data when LOG_LENGTH = 0

core: Don't generate logic for log data when LOG_LENGTH = 0

This adds "if LOG_LENGTH > 0 generate" to the places in the core
where log output data is latched, so that when LOG_LENGTH = 0 we
don't create the logic to collect the data which won't be stored.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>